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.

src/testdir/test_gui_init.vim

@@ -2,9 +2,8 @@
 " startup to take effect at runtime.
 
 source shared.vim
-if !CanRunGui()
-  throw 'Skipped: cannot run GUI'
-endif
+source check.vim
+CheckCanRunGui
 
 source setup_gui.vim