Tests to support OFFLOAD_LOGDIR on GCS

[nj1973]

Issue #177 added support for logs to GCS, however it did not add support for running our tests with OFFLOAD_LOGDIR set to a GCS location. This was because another issue had changed most of the tests and this work also required a mass edit to the same files.

I decided it made sense to postpone this part, seeing as it is internal to developer processes, to avoid a lot of unnecessary chore.

To do:

• Done: Change all tests that use get_test_messages to use the context manager version instead, get_test_messages_ctx
• Up to now we wanted tests to include child offloads in the same log file as the test, this doesn't work for GCS logs, we need the offload log to be complete and closed so the test can open and interrogate it. Need to review how to do this in scenario_runner.run_offload.