[ticket/9990] Add docs for running slow tests

PHPBB3-9990

tests/RUNNING_TESTS.txt

@@ -50,6 +50,15 @@ Once the prerequisites are installed, run the tests from the project root direct
 
     $ phpunit
 
+Slow tests
+--------------
+Certain tests, such as the UTF-8 normalizer or the DNS tests tend to be slow.
+Thus these tests are in the `slow` group, which is excluded by default. You can
+enable slow tests by copying the phpunit.xml.all file to phpunit.xml. If you only
+want the slow tests, run:
+
+    $ phpunit --group slow
+
 More Information
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