Merge #17228: test: Add RegTestingSetup to setup_common

fa0a731d007a8e76d2740a2e6ead2289de77e475 test: Add RegTestingSetup to setup_common (MarcoFalke)
fa54b3e2485f701aa420c37f09a2859a5b805161 test: move-only ComputeFilter to src/test/lib/blockfilter (MarcoFalke)

Pull request description:

  The default chain for `TestingSetup` is the main chain. However, any test that wants to mine blocks on demand needs to switch to regtest. This is done manually and in-line right now.

  Fix that by creating an explicit `RegTestingSetup` and use it where appropriate.

  Also, add a move-only commit to move `ComputeFilter` into the newly created unit test library.

  Both commits are part of #15845, but split up because they are useful on their own.

ACKs for top commit:
  practicalswift:
    ACK fa0a731d007a8e76d2740a2e6ead2289de77e475 -- diff looks correct

Tree-SHA512: 02b9765580b355ed8d1be555f8ae11fa6e3d575f5cb177bbdda0319378837e29de5555c126c477dc8a1e8a5be47335afdcff152cf2dea2fbdd1a988ddde3689b

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