Revert "[gn] use relative paths for gtest and gmock"

This reverts commit 131f4a30.

Reason for revert: Breaks snapshot builder: https://ci.chromium.org/p/v8/builders/ci/Linux%20Snapshot%20Builder/16103

Original change's description:
> [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/1498473
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Yang Guo <yangguo@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#59996}

TBR=machenbach@chromium.org,yangguo@chromium.org

Change-Id: Ia2f377aa2e0fc69206104c4942085a9ded4534e2
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8941
Reviewed-on: https://chromium-review.googlesource.com/c/1497077
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60000}

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) {