test: mark test-signal-unregister as flaky

This test just failed on Ubuntu in Jenkins, for a change that
is 100% Windows-specific.

Reviewed-By: Joao Reis <reis@janeasystems.com>
PR-URL: https://github.com/joyent/node/pull/25750

test/simple/simple.status

@@ -10,6 +10,7 @@ test-timers-first-fire            : PASS,FLAKY
 test-http-pipeline-flood          : PASS,FLAKY
 
 [$system==linux]
+test-signal-unregister            : PASS,FLAKY
 
 [$system==macos]
 test-fs-watch                     : PASS,FLAKY