Register the "slow" and "incremental" marks with pytest

Pytest was printing warnings that the slow and incremental marks were
not registered.

Change-Id: Ie8665f469be75c8248a4d65c2fb9954f6d93b115
diff --git a/setup.cfg b/setup.cfg
index 1b1ff54..de7d3d2 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -3,3 +3,5 @@
 
 markers =
   docker: Tests that require to run and interact with a docker container
+  incremental: Test classes containing tests that need to run incrementally
+  slow: Tests that run slower than the average test