RFR: 8327835: Convert java/awt/FileDialog/RegexpFilterTest/RegexpFilterTest applet test to main

[Alexander Zvegintsev]

The `java/awt/FileDialog/RegexpFilterTest/RegexpFilterTest` test is written to test the very specific functionality of `XFileDialogPeer`, so it shouldn't be run on platforms other than Linux.

We also need to set `sun.awt.disableGtkFileDialogs` to `true` to be able to test `XFileDialogPeer`, because by default it tries to use the Gtk file dialog if it is available.
The Gtk file dialog has a search field hidden under the magnifying glass icon, but it has a different syntax.

The Windows system file dialog allows you to use wildcard filtering in the "File name" field by hitting the `enter` afterwards, but again, this test was written to test our own file dialog implementation, not the system's.

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

Commit messages:
 - fix spacing
 - remove def timeout
 - remove separate dir
 - initial

Changes: https://git.openjdk.org/jdk/pull/18203/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18203&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327835
  Stats: 345 lines in 4 files changed: 75 ins; 270 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18203.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18203/head:pull/18203

PR: https://git.openjdk.org/jdk/pull/18203