Merge bitcoin/bitcoin#23420: test: Correct MyPy typing for subtest decorator

467fe5779ca85bd8d58fde65948ffc8c35078e60 test: Correct MyPy typing for subtest decorator (Pavel Safronov)

Pull request description:

  This is the part of the effort to make python typing correct https://github.com/bitcoin/bitcoin/issues/19389

  The typing of the `subtest` decorator within `p2p_segwit.py` test file was incorrect.

  Since `subtest` function is defined as a member of the class, it expects `self` as a first argument, and it is not provided. Hence the typing errors (that are currently suppressed by `type: ignore`).

  ```
  (venv) vagrant@ubuntu-focal:/vagrant/test/functional$ mypy p2p_segwit.py
  p2p_segwit.py:298: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:327: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:358: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:447: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:519: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:561: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:659: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:670: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:737: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:826: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:866: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:941: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:977: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1052: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1089: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1136: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1220: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1312: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1406: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1440: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1543: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1729: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1782: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1881: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:1983: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  p2p_segwit.py:2027: error: Argument 1 to "subtest" has incompatible type "Callable[[SegWitTest], Any]"; expected "SegWitTest"
  Found 26 errors in 1 file (checked 1 source file)
  ```

  However, the tests are passing, because there is no `self` argument passed when it is called as a decorator.

  There is also suppressed pylint error `# noqa: N805` pointing to the same issue.
  ```
  N805 first argument of a method should be named 'self'
  ```

  So the solution is to move the `subtest` definition outside the class, so the `self` argument is no longer required.

  After doing so, both mypy and unittests are successfully passing:

  ```
  (venv) vagrant@ubuntu-focal:/vagrant/test/functional$ mypy p2p_segwit.py
  Success: no issues found in 1 source file
  ```

  ```
  (venv) vagrant@ubuntu-focal:/vagrant/test/functional$ ./test_runner.py p2p_segwit
  Temporary test directory at /tmp/test_runner__🏃_20211103_011449
  Running Unit Tests for Test Framework Modules
  ..........
  ----------------------------------------------------------------------
  Ran 10 tests in 0.546s

  OK
  Remaining jobs: [p2p_segwit.py]
  1/1 - p2p_segwit.py passed, Duration: 81 s

  TEST          | STATUS    | DURATION

  p2p_segwit.py | ✓ Passed  | 81 s

  ALL           | ✓ Passed  | 81 s (accumulated)
  Runtime: 81 s
  ```
  ```

ACKs for top commit:
  fanquake:
    ACK 467fe5779ca85bd8d58fde65948ffc8c35078e60

Tree-SHA512: e4c3e2d284f47a6bfbf4af22d4021123cdd9c2ea16ec90a91b466ad1a5af615bb4e15959e6cf56c788701d7e7cbda91a8ffc4347965095c3384eae3d28f261af

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