test: mark microtask-queue-run(-domain) as flaky

test-microtask-queue-run and test-microtask-queue-run-domain fail very
rarely at least on linux x64, windows 2012r2 x64 and smartos x86

Reviewed-By: Alexis Campailla <alexis@janeasystems.com>
PR-URL: https://github.com/joyent/node/pull/25606

test/simple/simple.status

@@ -3,6 +3,8 @@ prefix simple
 test-crypto-domains               : PASS,FLAKY
 test-debug-signal-cluster         : PASS,FLAKY
 test-cluster-basic                : PASS,FLAKY
+test-microtask-queue-run          : PASS,FLAKY
+test-microtask-queue-run-domain   : PASS,FLAKY
 
 [$system==win32]
 test-timers-first-fire            : PASS,FLAKY