test: Tool wallet test coverage for unexpected writes to wallet

This commit adds test coverage in `test/functional/tool_wallet.py` to reproduce unexpected writes to the wallet as described in https://github.com/bitcoin/bitcoin/issues/15608:

    - wallet tool `info` unexpectedly writes to the wallet file if the wallet file permissions are read/write.

    - wallet tool `info` raises with "Error loading . Is wallet being used by another process?" if the wallet file permissions are read-only.

1. Reproduce the reported issue, define the current unexpected behavior, and add test coverage to guide a future fix in the form of commented-out assertions to be uncommented when testing/fixing.

2. Provisionally extend the same coverage to the wallet tool create test and the getwalletinfo test as regression tests while fixing the issue.

3. Add some logging for sanity checking.

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Changes after rebase:

5. Make wallet_path an instance method instead of a function in tool_wallet.py as per Marco Falke review suggestion.

6. Assert wallet permissions instead of logging them in tool_wallet.py. This ran into an issue with Appveyor keeping permissions at 666 so allowed for 666 as a workaround.

7. Change the added logging from info to debug level.

8. More helpful assertions order in tool_wallet.py#assert_tool_output. This change makes #assert_tool_output raise "Error loading wallet.dat. Is wallet being used by another process?" rather than a less-helpful message when debugging the read-only wallet permissions issue.

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