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_lua.vim b/src/testdir/test_lua.vim
index 604cfed..5d2f088 100644
--- a/src/testdir/test_lua.vim
+++ b/src/testdir/test_lua.vim
@@ -1,7 +1,7 @@
 " Tests for Lua.
 
 if !has('lua')
-  finish
+  throw 'Skipped, lua feature missing'
 endif
 
 func TearDown()