assertions.rb: fix result of assert_nothing_raised

* test/lib/test/unit/assertions.rb (assert_nothing_raised): do not
  discard the result of the given block.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54142 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/assertions.rb

@@ -186,7 +186,6 @@ module Test
             raise
           end
         end
-        nil
       end
 
       # :call-seq: