doc: link to common docs in test writing guide

PR-URL: #13118
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <mhdawson@ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/guides/writing-tests.md

@@ -56,7 +56,7 @@ const common = require('../common');
 The first line enables strict mode. All tests should be in strict mode unless
 the nature of the test requires that the test run without it.
 
-The second line loads the `common` module. The `common` module is a helper
+The second line loads the `common` module. The [`common` module][] is a helper
 module that provides useful tools for the tests.
 
 Even if a test uses no functions or other properties exported by `common`,
@@ -340,3 +340,4 @@ will depend on what is being tested if this is required or not.
 
 [Google Test]: https://github.com/google/googletest
 [Test fixture]: https://github.com/google/googletest/blob/master/googletest/docs/Primer.md#test-fixtures-using-the-same-data-configuration-for-multiple-tests
+[`common` module]: https://github.com/nodejs/node/blob/master/test/common/README.md