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_signals.vim b/src/testdir/test_signals.vim
index 5dec1d3..365958a 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -1,7 +1,7 @@
 " Test signal handling.
 
 if !has('unix')
-  finish
+  throw 'Skipped, not on Unix'
 endif
 
 source shared.vim