test: mark test failing on AIX as flaky

Checklist

• make -j4 test (UNIX), or vcbuild test nosign (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test

Description of change

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 https://github.com/nodejs/node/pull/7564 test-async-wrap-post-did-throw test-async-wrap-throw-from-callback test-crypto-random
• being worked under https://github.com/nodejs/node/issues/7973 test-stdio-closed
• covered by https://github.com/nodejs/node/issues/3796 test-debug-signal-cluster