The regression tests use test_xfail() (exit 78) to assert a known, documented
residual on platforms where the fix can't apply -- e.g. link-dest-relative-basis
XFAILs where the receiver has no openat2/O_RESOLVE_BENEATH and the portable
resolver rejects the '..' for safety. runtests.py counted exit 78 in the
generic else->failed branch, so a bare XFAIL failed the whole suite; tally it
separately ('N xfailed (expected)') and exclude it from the failure exit code.
Also add --race-timeout plumbing (race_timeout env) for race tests.
