Merge from Widevine repo of http://go/wvgerrit/59700
This CL sets the environment variable MODEL_NAME to be the current
unit test. When running on a test platform, this environment variable
is used in the license request as model_name.
test: unit tests.
bug: 38004627
Change-Id: I347c5cec35942d68285cc01615b976097f37d214