MAINT: Added xfail_strict option.

pytest.ini

-[pytest]
-markers =
-    local_server: mark a test as requiring a local caosdb server
-    slow: mark a test as slow (execution regularly takes 5s or longer)

tox.ini

@@ -17,3 +17,9 @@ commands_pre=pip install ../caosdb-pylib/
     python -c "import caosdb; print(caosdb.version.version)"
 # Add "-x" to stop at first error.
 commands=pytest --cov=caosdb -vv {posargs}
+
+[pytest]
+markers =
+    local_server: mark a test as requiring a local caosdb server
+    slow: mark a test as slow (execution regularly takes 5s or longer)
+xfail_strict = True