test: per-run --basetemp so concurrent worktrees don't share pytest tmp
The existing TMPDIR=/var/tmp comment claimed pytest's default per-run
isolation prevented collisions, but multiple worktrees running pytest
concurrently still share /var/tmp/pytest-of-$USER/pytest-N/ — pytest wipes
the basetemp on startup, deleting in-flight tmp_path dirs from sibling
runs and producing FileNotFoundError storms. Add a unique --basetemp
(mktemp -d) to every top-level Makefile pytest invocation, and pass an
explicit --basetemp to the nested pytest canary in test_tmpdir_fallback.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>