[cppgc] Fix dependency for cppgc tests

Add a dependency to a cppgc target for the the cppgc unit tests sources
so that the header files are used correctly. Previously it was working
because it added the external config which sets up the include
directories correctly, but would fail if the v8-gn.h file was not
generated quickly enough or if the cppgc_unittests_sources was built on
its own.

Bug: v8:11292
Change-Id: If12be4809b59b8dd5705468ad0343a1118547092
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2640458Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72222}

test/unittests/BUILD.gn

@@ -134,8 +134,13 @@ v8_source_set("cppgc_unittests_sources") {
     "//testing/gtest",
   ]
 
-  if (!cppgc_is_standalone) {
-    deps += [ "../..:v8_tracing" ]
+  if (cppgc_is_standalone) {
+    deps += [ "../..:cppgc_for_testing" ]
+  } else {
+    deps += [
+      "../..:v8_for_testing",
+      "../..:v8_tracing",
+    ]
   }
 }