Adds config test for required kernel modules

The u2mfn kernel module, built by dkms, is required for graphical
support in VMs. Let's ensure that the kernel object is present for the
active kernel, otherwise graphical displays will be garbled.

tests/test_vms_exist.py

@@ -36,6 +36,12 @@ def _check_kernel(self, vm):
         assert kernel_version.endswith("-grsec-workstation")
         assert kernel_version == EXPECTED_KERNEL_VERSION
 
+        u2mfn_filepath = "/usr/lib/modules/{}/updates/dkms/u2mfn.ko".format(EXPECTED_KERNEL_VERSION)
+        # cmd will raise exception if file not found
+        stdout, stderr = vm.run("sudo test -f {}".format(u2mfn_filepath))
+        assert stdout == b""
+        assert stderr == b""
+
     def _check_service_running(self, vm, service):
         """
         Ensures a given service is running inside a given VM.