test: try to reduce flakes on Windows

• test: always suffix tmpdir This makes temp dir names consistent whether we run in stand-alone mode, via test.py in single process, or in multi-process.
• test: shell out to rmdir first on Windows cmd's rmdir is hardened to deal with Windows edge cases, like lingering processes, indexing, and AV checks. So we give it a try first.
• test: regression test tmpdir

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines