patch 8.1.1524: tests are silently skipped

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.
diff --git a/src/testdir/test_syntax.vim b/src/testdir/test_syntax.vim
index bdcef51..8d9d7d3 100644
--- a/src/testdir/test_syntax.vim
+++ b/src/testdir/test_syntax.vim
@@ -1,7 +1,7 @@
 " Test for syntax and syntax iskeyword option
 
 if !has("syntax")
-  finish
+  throw 'Skipped, syntax feature missing'
 endif
 
 source view_util.vim