patch 8.1.1544: some balloon tests don't run when they can

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.

src/testdir/test_cscope.vim

@@ -1,10 +1,11 @@
 " Test for cscope commands.
 
-if !has('cscope') || !has('quickfix')
-  throw 'Skipped, cscope or quickfix feature missing'
-endif
+source check.vim
+CheckFeature cscope
+CheckFeature quickfix
+
 if !executable('cscope')
-  throw 'Skipped, cscope program missing'
+  throw 'Skipped: cscope program missing'
 endif
 
 func CscopeSetupOrClean(setup)