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

[Sergey Bylokhov]

On Fri, 29 Aug 2025 08:30:21 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

>The purpose of this PR is to avoid running GTK L&F specific tests on the platforms where GTK L&F isn't available.

But why not try loading it? That would help verify that the validation for missing dependencies works correctly.
I think the patch is based on assumptions that are not necessarily true:
 - GTK L&F can be supported on platforms other than Linux
 - headful systems always have the GTK library installed

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PR Review Comment: https://git.openjdk.org/jdk/pull/26995#discussion_r2311763763