improve test/readme

test/README.md

 # Tests
 
-There are two types of tests: "testinfra" tests, and "behave" tests. "Testinfra"
-tests run on the OAS machine and test if the infrastructure of the machine is
-setup correctly. They also test the validity of TLS certificates. The
-"behave" tests run in a browser and test if all the interfaces are still up and
-running.
+There are two types of tests: "testinfra" tests, and "behave" tests.
+
+- Testinfra tests are split into two groups, lets call them blackbox- and
+  whitebox tests.  The blackbox tests run on your laptop and test the OAS
+  cluster from the outside. For example, the certificate check will check if the
+  OAS will return valid certificates for the provided services.  The whitebox
+  tests run on the OAS host and check i.e. if docker is installed in the right
+  version etc.
+- Behave tests run in a browser and test if all the interfaces are up and
+  running and correctly connected to each other.
 
 ## Run *testinfra* tests