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_memory_usage.vim

@@ -1,15 +1,15 @@
 " Tests for memory usage.
 
-if !has('terminal')
-  throw 'Skipped, terminal feature missing'
-endif
+source check.vim
+CheckFeature terminal
+
 if has('gui_running')
-  throw 'Skipped, does not work in GUI'
+  throw 'Skipped: does not work in GUI'
 endif
 if execute('version') =~# '-fsanitize=[a-z,]*\<address\>'
   " Skip tests on Travis CI ASAN build because it's difficult to estimate
   " memory usage.
-  throw 'Skipped, does not work with ASAN'
+  throw 'Skipped: does not work with ASAN'
 endif
 
 source shared.vim
@@ -20,7 +20,7 @@ endfunc
 
 if has('win32')
   if !executable('wmic')
-    throw 'Skipped, wmic program missing'
+    throw 'Skipped: wmic program missing'
   endif
   func s:memory_usage(pid) abort
     let cmd = printf('wmic process where processid=%d get WorkingSetSize', a:pid)
@@ -28,13 +28,13 @@ if has('win32')
   endfunc
 elseif has('unix')
   if !executable('ps')
-    throw 'Skipped, ps program missing'
+    throw 'Skipped: ps program missing'
   endif
   func s:memory_usage(pid) abort
     return s:pick_nr(system('ps -o rss= -p ' . a:pid))
   endfunc
 else
-  throw 'Skipped, not win32 or unix'
+  throw 'Skipped: not win32 or unix'
 endif
 
 " Wait for memory usage to level off.