cmake: Improve testing.

- 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
diff --git a/README.md b/README.md
index 8c0b94e..f5446f9 100644
--- a/README.md
+++ b/README.md
@@ -248,11 +248,11 @@
         -DENABLE_CCACHE=1 \
         -DAOM_TARGET_CPU=generic \
         -DENABLE_DOCS=0 \
+        -DENABLE_TESTS=0 \
         -DCONFIG_ACCOUNTING=1 \
         -DCONFIG_INSPECTION=1 \
         -DCONFIG_MULTITHREAD=0 \
         -DCONFIG_RUNTIME_CPU_DETECT=0 \
-        -DCONFIG_UNIT_TESTS=0 \
         -DCONFIG_WEBM_IO=0 \
         -DCMAKE_TOOLCHAIN_FILE=path/to/emsdk-portable/.../Emscripten.cmake
 ~~~