RFR: 8233648: [TESTBUG] DefaultMenuBarTest.java failing on macos

[Alexander Zvegintsev]

Looks like this test does not fail anymore.

It does not fail once even without changes, but I've added extra `waitForIdle()` just for safety.

Checked on ~350 test runs.

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Commit messages:
 - initial

Changes: https://git.openjdk.java.net/jdk/pull/6015/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6015&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8233648
  Stats: 3 lines in 2 files changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6015.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6015/head:pull/6015

PR: https://git.openjdk.java.net/jdk/pull/6015