Extend tox.ini with testpaths and {posargs} in poetry-pytest invocation

[chrisinmtown]

This allows limiting the scope of the test run, which can save much time.

Run one test in one test environment:

tox -e py39 -- tests/test_api.py::test_api

Run one test in all test environments:

tox -- tests/test_api.py::test_api

Run all tests in one file in one test environment:

tox -e py39 -- tests/test_api.py

Run all tests in one file in all test environments:

tox -- tests/test_api.py

[chrisinmtown]

I'm impatient :) and don't like waiting for all 802 tests to run in all 6 test environments. This change makes it possible for faster code-test cycles. Please consider, comment, etc., thank you.

[coveralls]

coverage: 94.292%. remained the same
when pulling 38384c4 on chrisinmtown:add-tox-tests-posargs
into 0bf9d1f on spec-first:main.

[chrisinmtown]

Greetings @Ruwann and @RobbeSneyders will you please review this contribution? Thanks.

[chrisinmtown]

@RobbeSneyders I see you were debugging a problem today with the tests in 3.14. This tiny change makes that easier by allowing you to run tests in only that environment. Please consider a merge here?

[RobbeSneyders]

Thanks @chrisinmtown

[chrisinmtown]

It looks like I didn't test this enough. As far as I can tell, when poetry invokes pytest, it ignores the options in the tox.ini file. So when I removed the argument "tests" from that invocation, it caused damage. I will put up a new PR to back out some of the damage that caused.

[chrisinmtown]

Please see a minor change to a problem introduced here: #2092