Move remaining test targets out of //mojo/public.

The test targets currently in //mojo/public depend on the EDK to run and thus
are not part of the public SDK. This CL converts the remaining test targets in
the public SDK into testonly mojo_sdk_source_sets, and adds new test targets
under the EDK that depend on the corresponding source_sets as well as the EDK
mechanism for running the unittests.

Note that the dependencies from the test targets to the source_sets are
specified via relative paths as part of the work toward having the EDK be
flexible wrt its location in a client repo.

Initial review: https://codereview.chromium.org/767083006/

BUG=439082
TBR=jamesr@chromium.org

Review URL: https://codereview.chromium.org/796853003
diff --git a/mojo/BUILD.gn b/mojo/BUILD.gn
index 494b624..ab86729 100644
--- a/mojo/BUILD.gn
+++ b/mojo/BUILD.gn
@@ -50,8 +50,8 @@
     "//mojo/edk/test:mojo_public_bindings_unittests",
     "//mojo/edk/test:mojo_public_environment_unittests",
     "//mojo/edk/test:mojo_public_system_perftests",
-    "//mojo/public/cpp/system/tests:mojo_public_system_unittests",
-    "//mojo/public/cpp/utility/tests:mojo_public_utility_unittests",
+    "//mojo/edk/test:mojo_public_system_unittests",
+    "//mojo/edk/test:mojo_public_utility_unittests",
     "//mojo/services/network:apptests",
     "//mojo/services/view_manager/public/cpp/tests:mojo_view_manager_lib_unittests",
     "//mojo/tools:message_generator",