Add CMake configuration for iree::testing

Closes #240

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/240 from marbre:cmake-testing 516e7a5f3ee24c2abb74f7f6231e7af747546827
PiperOrigin-RevId: 287355185
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f71ec76..c714526 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -94,6 +94,10 @@
   add_subdirectory(build_tools/third_party/dear_imgui EXCLUDE_FROM_ALL)
 endif()
 
+if(${IREE_BUILD_TESTS})
+  add_subdirectory(third_party/benchmark EXCLUDE_FROM_ALL)
+endif()
+
 #-------------------------------------------------------------------------------
 # IREE top-level libraries
 #-------------------------------------------------------------------------------
@@ -104,6 +108,7 @@
 add_subdirectory(iree/hal)
 add_subdirectory(iree/rt)
 add_subdirectory(iree/schemas)
+add_subdirectory(iree/testing)
 add_subdirectory(iree/vm)
 
 if(${IREE_BUILD_COMPILER})