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Right now DeviceTest is actually two main tests (ndk-build and cmake),
each with subtests. To ease splitting the build step and run step
into separate pieces for better parallelization (and just as a
general cleanup), split them into two test types.

To facilitate this, make test scanners more intelligent. We're no
longer requiring that every directory in the test suite is a test
(empty directories used to cause weird errors when we tried to
ndk-build them), and each directory can now become multiple tests.
This is useful for splitting up the ndk-build tests from the cmake
tests.

As an added bonus, the new BuildTestScanner and DeviceTestScanner can
have multiple configurations added. This paves the way for
parallelizing things like validate.py at the highest level so we only
have to invoke the test runner once, meaning the impact of long tests
will be less significant since they can all be run in parallel.

Test: ./validate.py
Bug: None
Change-Id: I66704486ffabc1f2d0dfb7bb3a6879f...