fix(3597): bug-1974 windows EPERM — retry rmSync on tmpDir teardown

Test spawns a fire-and-forget subprocess; on Windows the child may still
hold a handle on tmpDir when afterEach runs, producing EPERM from
fs.rmSync. Match helpers.cleanup() retry budget (20 × 250ms = 5s) to
absorb the deferred-handle window without pulling in the helper (this
suite predates it).

Closes the Windows test (windows-latest, 24/26) lane failure on #3649.