test: add extended test for triggering mount rate limit

It's hard to trigger the failure to exit the rate limit state in
isolation as it needs multiple event sources in order to show that it
gets stuck in the queue. Hence why this is an extended test.

test/TEST-60-MOUNT-RATELIMIT/Makefile

@@ -0,0 +1 @@
+../TEST-01-BASIC/Makefile

test/TEST-60-MOUNT-RATELIMIT/test.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -e
+TEST_DESCRIPTION="Test that mount/unmount storms can enter/exit rate limit state and will not leak units"
+
+. $TEST_BASE_DIR/test-functions
+
+do_test "$@"