Use "os.Rename()" in tests instead of "mv"

Use "os.Rename()" in tests instead of "mv"

This was added in 00f2dfd (2011): "BSD Test: Need to use outside program
to trigger some events". All tests pass fine with just os.Rename(), so
just use that.

Might add another real-world test in the future for common tools like
mv, cp, vi, etc. because they don't always behave the same as library
calls, but calling mv in *only* the mv() helper is rather odd.

This also renames a few things in the kqueue helper for clarity, and
uses map[string]struct{} if we just use them for lookups to an
allocation (I originally tried to fix #488, but gave up on that for now,
hence the kqueue changes).