Re-enable a SubMesh unit test

[Prof Garth Wells]

A unit test in

test/unit/mesh/python/SubMesh.py

was accidentally disabled. Needs to be got running again.

[Prof Garth Wells]

Fix committed.

[Johan Hake]

Nice! Looks like there still is an out-commented test.

[Prof Garth Wells]

There is some old commented code (which might be re-used with a new MeshFunction constructor) , but the test is there (testing the transfer of makers from a Mesh to a SubMesh).

[Johan Hake]

Yes, I saw that. It was not that test I was referring to.

[Prof Garth Wells]

Are you going to tell me which one you're referring to?

[Johan Hake]

xtest_creation -> test_creation

[Prof Garth Wells]

Fixed in next.