test: Fix type of MachineCase.machine_class

It's not NoneType. anaconda-webui sets a different class, and mypy
complains:

> test/anacondalib.py:38: error: Incompatible types in assignment (expression has type "type[VirtInstallMachine]", variable has type "None")  [assignment]

test/common/testlib.py

@@ -1332,7 +1332,7 @@ class MachineCase(unittest.TestCase):
     runner = None
     machine: testvm.Machine
     machines: Mapping[str, testvm.Machine]
-    machine_class = None
+    machine_class: type | None = None
     browser: Browser
     network = None
     journal_start: str | None = None