RFR: 8225777: java/awt/Mixing/MixingOnDialog.java fails on Ubuntu

[Alexander Zvegintsev]

Looks like the test started failing after increasing of window's title height in Ubuntu theme:

![image](https://user-images.githubusercontent.com/77687766/138088503-f380bcb2-1ffc-4d58-b6b2-2efc56b7975c.png)
So the click happens on windows title instead of the button.

For example, macOS has smaller window title height, which allows test to pass.
<img src="https://user-images.githubusercontent.com/77687766/138088659-bd636518-c23c-4c5e-bf97-61035753c71a.png" width="50%" height="50%">

Fix moves buttons away from window title.
![image](https://user-images.githubusercontent.com/77687766/138088880-a9d1ec38-cde2-437e-80b9-5b7bdf73fd6b.png)

Testing is green.

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Commit messages:
 - initial

Changes: https://git.openjdk.java.net/jdk/pull/6040/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6040&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8225777
  Stats: 32 lines in 2 files changed: 5 ins; 15 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6040.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6040/head:pull/6040

PR: https://git.openjdk.java.net/jdk/pull/6040