RFR: 8208565: [TEST_BUG] javax\swing\PopupFactory\6276087\NonOpaquePopupMenuTest.java throws NPE

[Prasanta Sadhukhan]

Test was failing in the past owing to NPE while accessing JMenu probably owing to fact the test did not wait for UI to be visible before starting the test, so added a delay after the frame is made visible. Also, added disposal of frame.
Also, it seems JDK-8213535 fix might be having a positive impact on this test.

Several iterations of this test in all platforms have passed (link in JBS).

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Commit messages:
 - 8208565: [TEST_BUG] javax\swing\PopupFactory\6276087\NonOpaquePopupMenuTest.java throws NPE
 - 8208565: [TEST_BUG] javax\swing\PopupFactory\6276087\NonOpaquePopupMenuTest.java throws NPE

Changes: https://git.openjdk.java.net/jdk/pull/8282/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8282&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8208565
  Stats: 84 lines in 2 files changed: 27 ins; 30 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8282.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8282/head:pull/8282

PR: https://git.openjdk.java.net/jdk/pull/8282