Add unittests target to the binary distribution (issue #1632)

- Move all tests from the top-level directory to tests/.
- Move files shared by cefclient and unittests to tests/shared/.
- Add a fused (single header/source file) version of gtest in
  tests/gtest/ with associated CMake configuration.
- Test-only headers are now exposed in include/test/. Unit test
  targets must define UNIT_TEST in order to access them.
- Replace usage of USING_CEF_SHARED with WRAPPING_CEF_SHARED for
  clarity (only the libcef_dll_wrapper target should define it).
- Remove the RENAME_DIRECTORY CMake macro which is no longer used.
- Remove C++11 usage from unittests sources for compatibility with
  the binary distribution configuration.
- Windows: Fix build errors due to chrome_elf.dll and imm32.lib
  missing from the CMake configuration.

include/capi/test/cef_translator_test_capi.h

@@ -38,6 +38,11 @@
 #define CEF_INCLUDE_CAPI_TEST_CEF_TRANSLATOR_TEST_CAPI_H_
 #pragma once
 
+#if !defined(BUILDING_CEF_SHARED) && !defined(WRAPPING_CEF_SHARED) && \
+    !defined(UNIT_TEST)
+#error This file can be included for unit tests only
+#endif
+
 #include "include/capi/cef_base_capi.h"
 
 #ifdef __cplusplus