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".

src/testdir/test_debugger.vim

@@ -22,7 +22,7 @@ endfunc
 " Debugger tests
 func Test_Debugger()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
 
   " Create a Vim script with some functions