test: mark test failing on AIX as flaky

We now have adequate AIX hardware to add AIX to
the regular regression runs.

However, there are a couple of failing tests even
though AIX was green at one point.  This PR
marks those tests as flaky so that we can add AIX
so that we can spot any new regressions without making
the builds RED

The tests are being worked under the following PRs

- being worked under #7564
test-async-wrap-post-did-throw
test-async-wrap-throw-from-callback
test-crypto-random

- being worked under #7973
test-stdio-closed

- covered by #3796
test-debug-signal-cluster

PR-URL: #8065
Reviewed-By: joaocgreis - João Reis <reis@janeasystems.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/parallel/parallel.status

@@ -26,3 +26,15 @@ test-debug-signal-cluster         : PASS,FLAKY
 [$system==aix]
 test-fs-watch-enoent                 : FAIL, PASS
 test-fs-watch-encoding               : FAIL, PASS
+
+# being worked under https://github.com/nodejs/node/pull/7564
+test-async-wrap-post-did-throw           : PASS, FLAKY
+test-async-wrap-throw-from-callback      : PASS, FLAKY
+test-crypto-random                       : PASS, FLAKY
+
+#being worked under https://github.com/nodejs/node/issues/7973
+test-stdio-closed                        : PASS, FLAKY
+
+#covered by  https://github.com/nodejs/node/issues/3796
+# but more frequent on AIX ?
+test-debug-signal-cluster                : PASS, FLAKY