Move our gtest_main.cc to a better spot

Change-Id: Icd5bd79efaa1f1db3622a52c87a95f7a0c9e7318
diff --git a/aos/build/externals.gyp b/aos/build/externals.gyp
index c4bbaa2..eac3247 100644
--- a/aos/build/externals.gyp
+++ b/aos/build/externals.gyp
@@ -129,7 +129,7 @@
       'type': 'static_library',
       'sources': [
         '<(externals)/gtest-<(gtest_version)/src/gtest-all.cc',
-        '<(AOS)/externals/gtest/gtest_main.cc',
+        '<(AOS)/testing/gtest_main.cc',
       ],
       'include_dirs': [
         '<(externals)/gtest-<(gtest_version)',
diff --git a/aos/externals/gtest/gtest_main.cc b/aos/testing/gtest_main.cc
similarity index 100%
rename from aos/externals/gtest/gtest_main.cc
rename to aos/testing/gtest_main.cc