RFR: 8328238: Convert few closed manual applet tests to main [v4]

[Alexey Ivanov]

On Thu, 21 Mar 2024 11:24:45 GMT, Prasanta Sadhukhan <psadhukhan at openjdk.org> wrote:

>> The tests were probably written before Java 5, even tests written later could still use `getContentPane` so that the test could be run with previous versions of Java. Yet it's irrelevant now.
>> 
>> The Java Tutorials have never been updated, [How to Make Frames](https://docs.oracle.com/javase/tutorial/uiswing/components/frame.html) uses `getContentPane` explicitly.
>
> Good point..this backward support still keep us using obsolete things..
> Java tutorials also there are few bugs like [6417303](https://bugs.openjdk.org/browse/JDK-6417303) which needs fix in the tutorial..dont know whom to ask or can we do it ourselves..

You're right. Cleaning up the code makes it easier to read.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18390#discussion_r1533716560