tests: register custom 'last' pytest mark to suppress warning

The test_tablets.py file uses @pytest.mark.last to ensure the
decommission test runs last. Register this mark in pyproject.toml
to eliminate the PytestUnknownMarkWarning.

Author: mykaul

pyproject.toml

@@ -121,6 +121,9 @@ log_level = "DEBUG"
 log_date_format = "%Y-%m-%d %H:%M:%S"
 xfail_strict = true
 addopts = "-rf"
+markers = [
+    "last: mark test to run last within its module group",
+]
 
 [tool.setuptools_scm]
 version_file = "cassandra/_version.py"