Modify Ethnologue tests to use registry redirection

On the build agents we set the environment variable
BUILDAGENT_SUBKEY. Unit tests should redirect the registry if they
see this variable. The Ethnologue unit tests didn't do this so far
which caused 6 tests to fail. Part of LT-13800.

This is a cherry-pick of e6e491.

Change-Id: I823b8395944202ae26f96efbb68893737161980f