added testament documentation link to tools.rst (#15481)

Author: enthus1ast

doc/tools.rst

@@ -30,7 +30,7 @@ The standard distribution ships with the following tools:
   | ``nimpretty`` is a Nim source code beautifier,
     to format code according to the official style guide.
 
-- | testament
+- | `testament <https://nim-lang.github.io/Nim/testament.html>`_
   | ``testament`` is an advanced automatic *unittests runner* for Nim tests,
     is used for the development of Nim itself, offers process isolation for your tests,
     it can generate statistics about test cases, supports multiple targets (C, JS, etc),