doc: added missing reference to test coverage info

Added a reference to test coverage doc since its useful to have it here
as well

PR-URL: #19825
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: mithunsasidharan

doc/guides/writing-tests.md

@@ -380,11 +380,17 @@ and tearing it down after the tests have finished.
 It also contains a helper to create arguments to be passed into Node.js. It
 will depend on what is being tested if this is required or not.
 
+### Test Coverage
+
+To generate a test coverage report, see the
+[Test Coverage section of the Pull Requests guide][].
+
 [ASCII]: http://man7.org/linux/man-pages/man7/ascii.7.html
 [Google Test]: https://github.com/google/googletest
 [Web Platform Tests Project]: https://github.com/w3c/web-platform-tests/tree/master/url
 [`common` module]: https://github.com/nodejs/node/blob/master/test/common/README.md
 [all maintained branches]: https://github.com/nodejs/lts
 [node.green]: http://node.green/
 [test fixture]: https://github.com/google/googletest/blob/master/googletest/docs/Primer.md#test-fixtures-using-the-same-data-configuration-for-multiple-tests
+[Test Coverage section of the Pull Requests guide]: https://github.com/nodejs/node/blob/master/doc/guides/contributing/pull-requests.md#test-coverage
 [directory structure overview]: https://github.com/nodejs/node/blob/master/test/README.md#test-directories