RFR: 8366369: Add @requires linux for GTK L&F tests

[Alexey Ivanov]

On Tue, 2 Sep 2025 16:45:24 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

> > GTK L&F may be supported on other platforms but _it is not_.
> 
> What's the point of testing on Windows and macOS? For the question above, it should be tested on Unix-like systems such as SAP or BSD.

It should be tested on Unix-like systems if and only if GTK library is supported.

> It is supported and certified on various systems, many of which do not include GTK. This is one of the reasons why failing to load the GTK library is not considered fatal.

If GTK library isn't supported (or available) on a system, I see no reason in wasting time and even starting the test. It's the purpose of `@requires` expression that can be amended to include other platforms where GTK library is supported and, therefore, GTK Look-and-Feel can be instantiated.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26995#discussion_r2316708683