Document bail-out()

Feature added to Rakudo in rakudo/rakudo@58dc8c6185

doc/Language/testing.pod6

@@ -548,6 +548,19 @@ required in the example to prevent the skipped tests from running.
 Alternatively, you could enclose the remaining tests in an C<else>
 branch.
 
+=item X<bail-out($reason?)|bail-out>
+
+If you already know the tests will fail, you can bail out of the test run
+using C<bail-out()>:
+
+=begin code
+    $has-db-connection or bail-out 'Must have database connection for testing';
+=end code
+
+The function aborts the current test run, signaling failure to the harness.
+Takes an optional reason for bailing out. The subroutine will call
+C<exit()>, so if you need to do a clean-up, do it before calling C<bail-out()>.
+
 =head1 Manual control
 
 If the convenience functionality documented above does not suit your needs,