tools/regression/priv: Don't call setgroups() with the effective GID

The 'setgroups(0, NULL);' stance works the same on all versions of
FreeBSD, getting rid of all supplementary groups.  The replaced code
would force the effective GID to also be a supplementary group.

Fixes:          9da2fe96ff ("kern: fix setgroups(2) and getgroups(2) to match other platforms")
MFC after:      5 days
Sponsored by:   The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D52289

tools/regression/priv/main.c

@@ -423,12 +423,10 @@ setup_file(const char *test, char *fpathp, uid_t uid, gid_t gid, mode_t mode)
 static void
 set_creds(const char *test, uid_t uid, gid_t gid)
 {
-	gid_t gids[1] = { gid };
-
 	if (setgid(gid) < 0)
 		err(-1, "test %s: setegid(%d)", test, gid);
-	if (setgroups(sizeof(gids)/sizeof(gid_t), gids) < 0)
-		err(-1, "test %s: setgroups(%d)", test, gid);
+	if (setgroups(0, NULL) < 0)
+		err(-1, "test %s: setgroups(NULL)", test);
 	if (setuid(uid) < 0)
 		err(-1, "test %s: seteuid(%d)", test, uid);
 }