pytest runner: use a container-local --basetemp so tmp_path works

[ArjunS07]

Purpose

Trials set TMPDIR to a folder mounted from the host. pytest puts its temporary root there (pytest-of-<user>), and when that folder belongs to another uid, pytest refuses it: every test that uses tmp_path or tmpdir errors at setup ("temporary directory ... is not owned by the current user"). This happened in the agent run and in the base run, so the regression check compared zero passing tests and could not find any regression.

Seen on memray#294 (all 12 trials), xarray#9770 (all 8; 4,700+ tests each), django-components#1538 (all 4) and 4 trials of dwave-cloud-client#709.

Changes

• template/tests/pytest_runner.py run_pytest_and_collect: when the caller gives no --basetemp, pass --basetemp=<new folder under /tmp> (inside the container).
  • Before: pytest used $TMPDIR/pytest-of-<user>.
• tests/docker/test_pytest_runner_regression.py: test_tmp_path_works_when_the_temp_root_belongs_to_another_user; fails without the change.

Verification

• tests/docker/test_pytest_runner_regression.py: 7 passed.

Notes

Stacked on #60.