RFR: 8356688: [TestBug] SWT tests fail on some Linux distros

[Jayathirth D V]

SWT unit tests were not enabled under [JDK-8169285](https://bugs.openjdk.org/browse/JDK-8169285) as it resulted in crash. We forced x11 as GDK backend under [JDK-8210411](https://bugs.openjdk.org/browse/JDK-8210411) but this is not sufficient in case where FXCanvas is embedded in an SWT window as captured at : https://github.com/openjdk/jfx/pull/1783#issuecomment-2844889943

Setting the environment variable in :swt:test gradle task fixes the issue. This is verified in Ubuntu 22.04 and 24.04 and swt tests run fine.

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Commit messages:
 - Don't fix 8340378 in this PR
 - Update comment
 - Set GDK_BACKEND as x11 in Linux

Changes: https://git.openjdk.org/jfx/pull/1890/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1890&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356688
  Stats: 11 lines in 1 file changed: 5 ins; 5 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/1890.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1890/head:pull/1890

PR: https://git.openjdk.org/jfx/pull/1890