[Bazel/C++] Split apart protobuf_test rule (#10039)

This splits the `protobuf_test` target into several other test targets, and adds `lite_unittest` and `lite_arena_unittest`, which were missing previously.

[Side note:] I did a pass over the new tests to thin their dependencies, and also split out `lite_test_util` from the existing `test_util`  target. I have left the `protobuf_test` target in place (but empty) for now... it is almost certainly safe to remove, but that can be done in a follow-up to reduce risk from this commit.