std.testing: make tmpDir more efficient

[andrewrk]

Extracted from #25592.

By integrating more with the test runner, std.testing.tmpDir can avoid a bunch of redundant work done by many unit tests.

The API can be made more convenient - give me a temporary file, give me a temporary directory, while having the test runner manage cleanup.

This can dovetail nicely with fuzzing by making the overhead code not part of the hot fuzzing loop.