[GTK] Migrate Skipped tests to test_expectations.txt

[GTK] Migrate Skipped tests to test_expectations.txt
https://bugs.webkit.org/show_bug.cgi?id=83768

Reviewed by Martin Robinson.

This will hopefully simplify gardening tasks. The tests without
bug entry have been marked with BUGWKGTK and the most problematic
suites (mostly for unimplemented or heavily unreliable features
(inspector)) have been SKIP flagged.

* platform/gtk/Skipped: Removed.
* platform/gtk/test_expectations.txt:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@113981 268f45cc-cd09-0410-ab3c-d52691b4dbfc