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

src/test/regress/pg_regress.c

@@ -330,7 +330,8 @@ signal_remove_temp(int signum)
 static const char *
 make_temp_sockdir(void)
 {
-	char	   *template = pg_strdup("/tmp/pg_regress-XXXXXX");
+	char	   *template = psprintf("%s/pg_regress-XXXXXX",
+									getenv("TMPDIR") ? getenv("TMPDIR") : "/tmp");
 
 	temp_sockdir = mkdtemp(template);
 	if (temp_sockdir == NULL)