[Tests] Add pytest markers to tox.ini

Remove pytest warnings due to unknown markers

tox.ini

@@ -11,6 +11,12 @@ minversion=3.0
 # Hide logged warnings and errors in test output.
 log_cli_level = CRITICAL
 addopts = -p no:warnings --basetemp=_pytest_temp
+markers =
+    todo: Tests that are yet to be written
+    gtkui: Tests for GTK code
+    security: Security related tests
+    slow: Tests that are particularly slow
+    internet: Tests that require internet connectivity
 
 # =================
 # Base dependencies