[ Merge of http://go/wvgerrit/150810 ]
This CL changes the call "Provision()" in some tests to use
"EnsureProvisioned()". The latter can actually call "Provision()" twice
in case of Prov 4 being used: first call to get oemcert.bin and second
call to get drm cert.bin.
Also added a file clean up during test tear-down.
Test: opk_ta with prov2 and prov4
Bug: 180530495
Bug: 236317198
Change-Id: I37c8708ead7f18e8b4d619bf6a080424e45406ee
1c96d290bd