RFR: 8366369: Add @requires linux for GTK L&F tests [v2]

[Alexey Ivanov]

> * Add `@requires os.family == "linux"` to prevent GTK L&F tests from running on other OS but Linux.
> * Let any exceptions from `UIManager.setLookAndFeel` propagate.
> * Remove `@author` tag.
> * Expand imports in `test/jdk/com/sun/java/swing/plaf/gtk/Test6635110.java`.
> * Add leading asterisks to jtreg comment block.

Alexey Ivanov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Clean up
   
   Declare paint method private, it's not overridden;
   Sort modifiers 'static final'; add @Override to run().
 - Require !(windows or mac) instead of linux
   
   Throw SkippedException if GTK L&F fails to install
 - Merge master
 - 8366369: Add @requires linux for GTK L&F tests

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26995/files
  - new: https://git.openjdk.org/jdk/pull/26995/files/9bac5761..37364e28

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26995&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26995&range=00-01

  Stats: 40326 lines in 1713 files changed: 21046 ins; 10927 del; 8353 mod
  Patch: https://git.openjdk.org/jdk/pull/26995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26995/head:pull/26995

PR: https://git.openjdk.org/jdk/pull/26995