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_xxd.vim b/src/testdir/test_xxd.vim
index 6b9dd54..63ac574 100644
--- a/src/testdir/test_xxd.vim
+++ b/src/testdir/test_xxd.vim
@@ -2,7 +2,7 @@
 if empty($XXD) && executable('..\xxd\xxd.exe')
   let s:xxd_cmd = '..\xxd\xxd.exe'
 elseif empty($XXD) || !executable($XXD)
-  finish
+  throw 'Skipped, xxd program missing'
 else
   let s:xxd_cmd = $XXD
 endif