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_utf8.vim b/src/testdir/test_startup_utf8.vim
index 8b3eca1..1b3d218 100644
--- a/src/testdir/test_startup_utf8.vim
+++ b/src/testdir/test_startup_utf8.vim
@@ -63,7 +63,7 @@
 
 func Test_detect_ambiwidth()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot run Vim in a terminal window'
   endif
 
   " Use the title termcap entries to output the escape sequence.