Add a small readme to the rgl test suite

lib/tests/readme.rst

@@ -0,0 +1,19 @@
+RGL tests
+=========
+
+This directory contains tests for the resource grammar library.
+
+Test cases
+----------
+
+Test cases are written as gf scripts. The file names must end with ".gfs" to
+be recognized as a script.
+
+The script are run using ``gf --run``.
+
+For a test to pass
+- gf must exit without an error code (exit 0)
+- the output must matchh the corresponding ``.out`` file
+
+For instance, if there is a test named ``my-test``, the gf commands should be
+in ``my-test.gfs`` and the expected output in ``my-test.out``