pw_transfer: Reduce duplication in test
pw_transfer: Reduce duplication in test

Move identical setup code to test fixtures. This reduces duplication and
keeps the unit tests focused on what they are intended to test.

Change-Id: I619876ebf9bf633d38ad93d1211cf8cc1e26374d
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/63641
Reviewed-by: Keir Mierle <69344103cc60b36d6e1fbdc31ce6504b2286c646@google.com>
Commit-Queue: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed