RFR: 8280820: Clean up bug8033699 and bug8075609.java tests: regtesthelpers aren't used

[Alexey Ivanov]

Removed references to `regtesthelpers` libarary which isn't used.
Organised imports and replaced anonymous classes with lambda expressions.

Put `waitForIdle` before the test execution starts and moved the frame to the centre of the screen.

The CI with the updated tests is green.

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Commit messages:
 - Put the frame in the centre of the screen
 - waitForIdle after creating the GUI
 - Typo: non-grouped
 - Use lambda expressions or method references instead of anonymous classes
 - No need to pass robot to hitKey
 - Use primitive boolean for ActionListener
 - Access mainFrame from EDT only
 - Removed regtesthelpers, organised imports, Java-style arrays in main

Changes: https://git.openjdk.java.net/jdk/pull/7322/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7322&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280820
  Stats: 75 lines in 2 files changed: 14 ins; 13 del; 48 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7322.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7322/head:pull/7322

PR: https://git.openjdk.java.net/jdk/pull/7322