Broken link under building and contributing for running tests

[carolinecullen]

Currently points to https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Running_Automated_JavaScript_Tests which is a broken link. I believe that we would want it to point to https://firefox-source-docs.mozilla.org/js/build.html#testing?

[carolinecullen]

or https://firefox-source-docs.mozilla.org/js/test.html

[jandem]

or https://firefox-source-docs.mozilla.org/js/test.html

Yeah I think this one is similar to the old page.