patch 8.1.1483: skipped tests are not properly listed

Problem:    Skipped tests are not properly listed.
Solution:   Throw a "Skipped" exception instead of using ":finish" or ":return".
diff --git a/src/testdir/test_diffmode.vim b/src/testdir/test_diffmode.vim
index 81913c8..e5a3f2f 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
@@ -748,7 +748,7 @@
 
 func Test_diff_screen()
   if !CanRunVimInTerminal() || !has('menu')
-    return
+    throw 'Skipped: cannot make screendumps and/or menu feature missing'
   endif
   " clean up already existing swap files, just in case
   call delete('.Xfile1.swp')
@@ -880,7 +880,7 @@
 
 func Test_diff_with_cursorline()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
 
   call writefile([
@@ -907,7 +907,7 @@
 
 func Test_diff_of_diff()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
 
   call writefile([