test: add test for specifying custom pidfile via `-pid`

[theStack]

This small PR adds test coverage for the -pid command line option, which allows to overrule the pid filename (bitcoind.pid by default). One can specify either a relative path (within the datadir) or an absolute one; the latter is tested using self.options.tmpdir. Note that the functional test file feature_init.py so far only contained a stress test; with this new sub-test added, both the description and the test name are adapted to be more generic.

[DrahtBot]

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Type	Reviewers
ACK	tdb3, ryanofsky, naiyoma, achow101

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[naiyoma]

Concept ACK +1 on the suggestion above to move BITCOIN_PID_FILENAME_DEFAULT and import it in both files.

This is out of scope for this PR, so feel free to ignore. it would be good to add an edge case to test starting a node when there's already a pre-existing PID file.

[theStack]

@tdb3:

"bitcoind.pid" is also mentioned in feature_filelock.py. Could add a commit that deduplicates this.

Created an example commit here: tdb3@b622390

Thanks! I've cherry-picked your commit, but reordered it to appear before the test introduction commit and changed the commit subject accordingly (introduce constant, instead of deduplicating). Let me know if that's okay for you.

@naiyoma:

This is out of scope for this PR, so feel free to ignore. it would be good to add an edge case to test starting a node when there's already a pre-existing PID file.

Good idea. I decided to leave that for a follow-up, happy to review if someone opens a PR.

[tdb3]

Thanks! I've cherry-picked your commit, but reordered it to appear before the test introduction commit and changed the commit subject accordingly (introduce constant, instead of deduplicating). Let me know if that's okay for you.

Looks great, thanks.

[achow101]

ACK 04e4d52420a0e6bf40d4bd6fe1f31f66db9eab0a