cppgc: Fix ODR violation in unittests target

V8 already depends on cppgc_base which means that unittests does not
need to depend on cppgc_for_testing any longer.

Move the cppgc_for_testing dependency to the stand-alone
cppgc_unittests binary

Bug: v8:10674
Change-Id: I07bfe30901eb1683d2e0ee0189f73b5244884f27
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2310249
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68970}

test/unittests/BUILD.gn

@@ -35,6 +35,7 @@ v8_executable("cppgc_unittests") {
 
   deps = [
     ":cppgc_unittests_sources",
+    "../..:cppgc_for_testing",
     "//testing/gmock",
     "//testing/gtest",
   ]
@@ -85,7 +86,6 @@ v8_source_set("cppgc_unittests_sources") {
   ]
 
   deps = [
-    "../..:cppgc_for_testing",
     "//testing/gmock",
     "//testing/gtest",
   ]