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.
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index cb27f58..08c079c 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -1,9 +1,8 @@
 " Tests for defining text property types and adding text properties to the
 " buffer.
 
-if !has('textprop')
-  throw 'Skipped, textprop feature missing'
-endif
+source check.vim
+CheckFeature textprop
 
 source screendump.vim