[gn] use relative paths for gtest and gmock

TBR=machenbach@chromium.org

Bug: v8:8941
Change-Id: I0bbd119f7a613785b6e5e01cd8e59d5de40d68e1
Reviewed-on: https://chromium-review.googlesource.com/c/1498473Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59996}

test/unittests/BUILD.gn

@@ -17,9 +17,9 @@ v8_executable("unittests") {
     "../..:v8_for_testing",
     "../..:v8_libbase",
     "../..:v8_libplatform",
+    "../../testing/gmock",
+    "../../testing/gtest",
     "//build/win:default_exe_manifest",
-    "//testing/gmock",
-    "//testing/gtest",
   ]
 
   data_deps = [
@@ -289,9 +289,9 @@ v8_source_set("unittests_sources") {
     "../..:v8_for_testing",
     "../..:v8_libbase",
     "../..:v8_libplatform",
+    "../../testing/gmock",
+    "../../testing/gtest",
     "//build/win:default_exe_manifest",
-    "//testing/gmock",
-    "//testing/gtest",
   ]
 
   if (is_win) {