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: #21781 PR-URL: #32189 Reviewed-By: Gireesh Punathil <[email protected]> Reviewed-By: Beth Griggs <[email protected]>

Author: MylesBorins

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