- Drop the CONFIG_ variables controlling test support.
  - Replace CONFIG_UNIT_TESTS with ENABLE_TESTS.
  - Replace CONFIG_DECODE_PERF_TESTS with ENABLE_DECODE_PERF_TESTS.
  - Replace CONFIG_ENCODE_PERF_TESTS with ENABLE_ENCODE_PERF_TESTS.
- Make proper options that are visible in the top-level
  CMakeLists.txt.
- Add an option for control of testdata download to allow
  disable of test data download when desired.
  - Set ENABLE_TESTDATA to 0 at CMake build generation time to disable
    creation of the testdata download targets.

Change-Id: I2bcf6978828c28dfedc8bca9fe74e87531518e14