test: mark test-timers-blocking-callback flaky on osx

This is only for 10.15 but this test is periodically failing across
many CI runs. Would like to mark this as flaky so we can avoid lots
of red CI.

Refs: nodejs#21781

test/sequential/sequential.status

@@ -15,6 +15,8 @@ test-worker-prof: PASS, FLAKY
 [$system==linux]
 
 [$system==macos]
+# https://github.com/nodejs/node/issues/21781
+test-timers-blocking-callback: PASS, FLAKY
 
 [$system==solaris] # Also applies to SmartOS