Split integrations tests into separate Bazel targets.

Currently all the tests are under the same Bazel target
`pull_replication_tests`.

With this new approach, all the `IT tests` will have its
own Bazel target, therefore the distributed build (RBE
backend) will improve the performance and the isolation
of failing tests will be easier.

Bug: Issue 16931
Change-Id: I22b43c968ec24de655bd8adbd45cdef8a440fa31