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_popupwin.vim b/src/testdir/test_popupwin.vim
index d9b0b29..a3f2046 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -1,8 +1,7 @@
 " Tests for popup windows
 
-if !has('textprop')
-  throw 'Skipped: textprop feature missing'
-endif
+source check.vim
+CheckFeature textprop
 
 source screendump.vim
 
@@ -515,7 +514,7 @@
 
 func Test_popup_time()
   if !has('timers')
-    throw 'Skipped, timer feature not supported'
+    throw 'Skipped: timer feature not supported'
   endif
   topleft vnew
   call setline(1, 'hello')
@@ -1176,7 +1175,7 @@
 
 func Test_notifications()
   if !has('timers')
-    throw 'Skipped, timer feature not supported'
+    throw 'Skipped: timer feature not supported'
   endif
   if !CanRunVimInTerminal()
     throw 'Skipped: cannot make screendumps'