Help: Add example for REQUIRED_FILES test property

Help/prop_test/REQUIRED_FILES.rst

@@ -1,7 +1,38 @@
 REQUIRED_FILES
 --------------
 
-List of files required to run the test.
+List of files required to run the test.  The filenames are relative to the
+test :prop_test:`WORKING_DIRECTORY` unless an absolute path is specified.
 
 If set to a list of files, the test will not be run unless all of the
 files exist.
+
+Examples
+~~~~~~~~
+
+Suppose that ``test.txt`` is created by test ``baseTest`` and ``none.txt``
+does not exist:
+
+.. code-block:: cmake
+
+  add_test(NAME baseTest ...)   # Assumed to create test.txt
+  add_test(NAME fileTest ...)
+
+  # The following ensures that if baseTest is successful, test.txt will
+  # have been created before fileTest is run
+  set_tests_properties(fileTest PROPERTIES
+    DEPENDS baseTest
+    REQUIRED_FILES test.txt
+  )
+
+  add_test(NAME notRunTest ...)
+
+  # The following makes notRunTest depend on two files. Nothing creates
+  # the none.txt file, so notRunTest will fail with status "Not Run".
+  set_tests_properties(notRunTest PROPERTIES
+    REQUIRED_FILES "test.txt;none.txt"
+  )
+
+The above example demonstrates how ``REQUIRED_FILES`` works, but it is not the
+most robust way to implement test ordering with failure detection.  For that,
+test fixtures are a better alternative (see :prop_test:`FIXTURES_REQUIRED`).