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_startup.vim b/src/testdir/test_startup.vim
index 0ce8c81..41bb12f 100644
--- a/src/testdir/test_startup.vim
+++ b/src/testdir/test_startup.vim
@@ -659,7 +659,7 @@
 
 func Test_start_with_tabs()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot make screendumps'
   endif
 
   let buf = RunVimInTerminal('-p a b c', {})