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This is a rough initial version. It makes several assumptions:
 - assumes each test source file is built into module with name equal
to basename of source file
 - assumes tests get installed into /data/nativetest

However, given those assumptions, both:
$ runtest --path <path to directory containing gtest source> and
$ runtest --path <path to single source file>
should work.

Coming soon: gtest host support

Bugs 4563370, 4584339

Change-Id: Ia42aeed7f9ee6402b0ceb7b5ccaaa66ac636fe49