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_syntax.vim b/src/testdir/test_syntax.vim
index 143f97a..bdcef51 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -536,7 +536,7 @@
 " Check highlighting for a small piece of C code with a screen dump.
 func Test_syntax_c()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot make screendumps'
   endif
   call writefile([
 	\ '/* comment line at the top */',