pg_regress: Observe TMPDIR

 pg_regress: Observe TMPDIR
 
 Put the temporary socket directory under TMPDIR, if that environment
 variable is set, instead of the hardcoded /tmp.
 
 This allows running the tests if there is no /tmp at all (for example
 on Windows, although running the tests with Unix-domain sockets is not
 enabled on Windows yet).  We also use TMPDIR everywhere else /tmp is
 hardcoded, so this makes the behavior consistent.
 
 Reviewed-by: Andrew Dunstan <andrew.dunstan@2ndquadrant.com>
 Discussion: https://www.postgresql.org/message-id/flat/54bde68c-d134-4eb8-5bd3-8af33b72a010@2ndquadrant.com