Add --tag-filters option to filter tests using execution_tag marker

[nprizal]

Test Engine Tags can be applied to a test using @pytest.mark.execution_tag(“key”, “value”). However, pytest built-in -m option doesn’t support filtering tests using marker arguments. It can filter tests that have any execution_tag markers, but it can't filter tests that have execution_tag marker with specific key and value.

This PR adds new custom options --tag-filters to filter tests based on execution_tag'skey & value.

# test_foobar.py
import pytest

@pytest.mark.execution_tag("color", "red")
def test_foo()
 assert True

@pytest.mark.execution_tag("color", "blue")
def test_bar()
  assert True

Then to filter tests with execution tag

$ pytest --tag-filters "color:blue" -v
=========================== test session starts ============================
platform darwin -- Python 3.13.2, pytest-9.0.2, pluggy-1.6.0
rootdir: /my/project
plugins: buildkite-test-collector-1.2.0
collected 2 items / 1 deselected / 1 selected

test_foobar.py::test_bar PASSED                                       [100%]

===================== 1 passed, 1 deselected in 0.1s =======================

[nprizal]

One of the tests I added failed in CI because numprocesses option doesn't exists in the subprocess environment. I added None as default to prevent error being raised when evaluation that option.