Add a test target with sharding support to the CMake build.
Two major things going on here: - One target is created for each file in test-data.sha1 (testdata_N), and the testdata target now depends on all testdata_N targets. The testdata build rule can now run with as many jobs as there are input files to speed up test data download. - GTest sharding support has been added to the tests via a runtests custom build target. First, the number of processors is detected, and then a custom target is created for each processor (test_N). Once each test_N target is created, the runtests custom target is created, and then made to depend on each test_N target. When CMake is unable to detect the number of processors 10 test targets are created. Each custom target then sets the GTEST_SHARD_INDEX and GTEST_TOTAL_SHARDS environment variables, allowing GTest to handle sharding internally. BUG=aomedia:76,aomedia:469 Change-Id: Ib6b7974932396fbf44b735d37155fa57561027ab