Add ref to testing document

Author: stoned

doc/Type/Test.pod6

@@ -8,6 +8,8 @@ This module provides a testing framework, and is used in the official suite that
 tests the specification. All its functions emit output conforming to the
 L<Test Anything Protocol|https://testanything.org>.
 
+Also see L<Writing and running tests in Raku|/language/testing>.
+
 =head1 Methods
 
 =head2 sub plan