Various unit tests were creating temporary files unconditionally in /tmp
and were not cleaning up after themselves. Introduce a new variant of
mkstemp(3p) that respects the TMPDIR environment variable, and use it in
the offending unit tests. This allows each test run to use a dedicated
TMPDIR that can be cleaned up after the run.
Changelog-None
Signed-off-by: Matt Whitlock <c-lightning@mattwhitlock.name>
1f79aad830