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_shortpathname.vim b/src/testdir/test_shortpathname.vim
index f151788..564cd96 100644
--- a/src/testdir/test_shortpathname.vim
+++ b/src/testdir/test_shortpathname.vim
@@ -2,7 +2,7 @@
 " Only for use on Win32 systems!
 
 if !has('win32')
-  finish
+  throw 'Skipped, not on MS-Windows'
 endif
 
 func TestIt(file, bits, expected)