Add testing instruction regarding govuk-content-schemas dependency

Author: erkde

doc/testing.md

@@ -5,6 +5,10 @@
 Some of the smart-answers tests require PhantomJS to be [installed on your machine
 natively](https://github.com/teampoltergeist/poltergeist/blob/master/README.md#installing-phantomjs).
 
+Smart-answers also require the govuk-content-schemas repository which can
+be [cloned](https://github.com/alphagov/govuk-content-schemas) into a sibling
+directory, or a directory referenced using GOVUK_CONTENT_SCHEMAS_PATH.
+
 ## Executing tests
 
 Run all tests by executing the following: