run_project_tests: fix assertion that crashes if you have git worktrees

git worktrees are frequently a handy tool to use. This glob pattern
asserts an error if the same named directory appears more than once,
which it will -- once per worktree -- but only because it is globbing
the entire project root, even though it should always only exclusively
care about files in `test cases/`. So just glob correctly, and then I
can run the testsuite locally again.

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