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_pyx3.vim b/src/testdir/test_pyx3.vim
index 4b6057a..1b7bdae 100644
--- a/src/testdir/test_pyx3.vim
+++ b/src/testdir/test_pyx3.vim
@@ -2,7 +2,7 @@
 
 set pyx=3
 if !has('python3')
-  finish
+  throw 'Skipped, python3 feature missing'
 endif
 
 let s:py2pattern = '^2\.[0-7]\.\d\+'