Add README on tests regarding single test execution

MR: https://gitlab.com/apparmor/apparmor/-/merge_requests/761

Signed-off-by: Georgia Garcia <georgia.garcia@canonical.com>
Acked-by: John Johansen <john.johansen@canonical.com>

utils/test/README.md

@@ -0,0 +1,18 @@
+# Running individual tests
+
+Python's unittest allows individual tests to be executed by specifying the class name and the test on the command line.
+When running tests individually, the unittest framework executes the "setUp" and "tearDown" methods automatically.
+For more information, refer to the [unittest documentation](https://docs.python.org/3/library/unittest.html).
+
+Make sure to set the environment variables pointing to the in-tree apparmor modules, and the in-tree libapparmor and its python wrapper:
+
+```bash
+$ export PYTHONPATH=..:../../libraries/libapparmor/swig/python/build/$(/usr/bin/python3 -c "import distutils.util; import platform; print(\"lib.%s-%s\" %(distutils.util.get_platform(), platform.python_version()[:3]))")
+$ export __AA_CONFDIR=.
+```
+
+To execute the test individually, run:
+
+```bash
+$ python3 ./test-tile.py ClassFoo.test_bar
+```