pw_i2c: Fix build issues with initiator mock

Other small cleanup with initiator mock. Also added the initiator mock
unit test into the pw_i2c test group.

Testing: Host test -- OK
Change-Id: If8397f1c8622d4bb5f371eb8322b1b16deb6fe2b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49341
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Kevin Zeng <zengk@google.com>