Move cmake build setup for testing into test/test.cmake.

- Move source list vars.
- Split source list vars into common/decoder/encoder sources.
- Move target definitions into function.
- Split targets into common/decoder/encoder targets.
- Update CMakeLists.txt to include test.cmake and call
  setup_aom_test_targets() at the appropriate time.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: Icd9ce67593c2de7ebd5c8ef921e31517b6d20945
2 files changed