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: 94.354%. remained the same
when pulling 861ab3d on chrisinmtown:add-tox-tests-posargs
into a1c53db on spec-first:main.