patch 8.1.1826: tests use hand coded feature and option checks

Problem:    Tests use hand coded feature and option checks.
Solution:   Use the commands from check.vim in more tests.
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index 97fd12b..985bbf4 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -1,9 +1,8 @@
 " Tests specifically for the GUI
 
 source shared.vim
-if !CanRunGui()
-  throw 'Skipped: cannot run GUI'
-endif
+source check.vim
+CheckCanRunGui
 
 source setup_gui.vim