Disable exclusive locking for cookie database for services_unittests

Because https://crbug.com/377940976 might cause background database
operations to continue after the network context has been destroyed
it is not possible to guarantee that the cookie files won't still
be locked during the services unittests.

Therefore, this CL disables exclusive locking for the cookie database
in these tests, so they do not flake on the bots, as well as
re-enabling the test.

A future CL will attempt to resolve the underlying issue.

BUG=377940976, 378060730

Change-Id: I8b9365a770c78221718f8017fbeb2eae16432035
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6013594
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Commit-Queue: Will Harris <wfh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1382748}