Keep nested pytester sessions isolated from pytest-randomly

[cpruijsen]

Nested testdir.runpytest sessions now pass -p no:randomly. That flag is a no-op when pytest-randomly is not installed.

Fixes #218.

Summary

• Nested testdir.runpytest sessions still auto-loaded pytest-randomly when that plugin was installed, even if the outer pytest was started with -p no:randomly. pytester drops PYTEST_ADDOPTS, and in-process pytest.main() re-autoloads entry points.
• test_run_session_teardown_once_after_reruns (and other teardown tests that assume collection order) failed because they assert default class/module/method order.
• Session and class fixtures already teardown once under shuffle; inner outcomes stayed 8 failed, 2 passed, 5 skipped, 1 error, 18 rerun. This is test isolation, not a product teardown fix.

What I chose and the alternative

Wrap testdir.runpytest so every nested session gets -p no:randomly.

The other options were making the teardown assertions order-independent, or passing the flag only on test_run_session_teardown_once_after_reruns. The reporter's outer -p no:randomly never reaches in-process nested sessions, and several tests need stable collection order (earlier vs later modules); some of those failures are seed-dependent. Disabling the plugin that reorders is the smallest reversible isolation.

Happy to switch to order-independent assertions, or to add pytest-randomly as a tox extra.

Test plan

• test_run_session_teardown_once_after_reruns fails with pytest-randomly installed and the wrap removed
• Same test still fails with outer -p no:randomly without the wrap
• Same test passes with the wrap, with and without outer -p no:randomly
• Full tests/ suite passes with pytest-randomly installed (several seeds)
• Full tests/ suite still passes with pytest-randomly uninstalled
• CI pytest matrix (8.2-9.1, xdist) once the PR is up

[icemac]

Thank you for this PR. 😃