tests: silence TSAN warning on test_seqlock exit

TSAN warns about leaving the second thread dangling.  Doesn't really
matter, but just add a pthread_join to get rid of the warning.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

tests/lib/test_seqlock.c

@@ -111,4 +111,5 @@ int main(int argc, char **argv)
 	writestr("main @release\n");
 	seqlock_release(&sqlo);
 	sleep(1);
+	pthread_join(thr1, NULL);
 }