NPE in UGI.getCurrentUser in ITestAbfsIdentityTransformer setup

Change logs of the files point some of the blame at me here; UGI.getCurrentUser is failing in ITestAbfsIdentityTransformer init just after a UGI.reset()

The public entry points in UGI should all call ensureInitialized().

I have no idea how this worked at all. I've been fiddling with ABFS accounts and tests which did work now cause NPEs in UGI.

This is all test setup; I could probably fix it in the ABFS test case simply by calling a UGI method we know calls ensureInitialized(). But fixing UGI is the right thing to do.