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

test/test_runner.cmake

diff --git a/test/test_runner.cmake b/test/test_runner.cmake
new file mode 100644
index 0000000..b8176d7
--- /dev/null
+++ b/test/test_runner.cmake
@@ -0,0 +1,20 @@
+##
+## Copyright (c) 2017, Alliance for Open Media. All rights reserved
+##
+## This source code is subject to the terms of the BSD 2 Clause License and
+## the Alliance for Open Media Patent License 1.0. If the BSD 2 Clause License
+## was not distributed with this source code in the LICENSE file, you can
+## obtain it at www.aomedia.org/license/software. If the Alliance for Open
+## Media Patent License 1.0 was not distributed with this source code in the
+## PATENTS file, you can obtain it at www.aomedia.org/license/patent.
+##
+if (NOT AOM_ROOT OR NOT AOM_CONFIG_DIR OR NOT AOM_TEST_TARGET
+    OR NOT GTEST_TOTAL_SHARDS OR "${GTEST_SHARD_INDEX}" STREQUAL "")
+  message(FATAL_ERROR
+          "The variables AOM_ROOT AOM_CONFIG_DIR AOM_TEST_TARGET
+          GTEST_SHARD_INDEX and GTEST_TOTAL_SHARDS must be defined.")
+endif ()
+
+set($ENV{GTEST_SHARD_INDEX} ${GTEST_SHARD_INDEX})
+set($ENV{GTEST_TOTAL_SHARDS} ${GTEST_TOTAL_SHARDS})
+execute_process(COMMAND ${AOM_CONFIG_DIR}/${AOM_TEST_TARGET})