Improvements to blocking test

[forsyth2]

Summary

Objectives:

• Streamline the blocking/non-blocking test.

Issue resolution:

• Further step towards [Feature]: Improve zstash testing #385, follow-up to Improve tests #393

Select one: This pull request is...

• a bug fix: increment the patch version
• a small improvement: increment the minor version
• a new feature: increment the minor version
• an incompatible (non-backwards compatible) API change: increment the major version

Small Change

• To merge, I will use "Squash and merge". That is, this change should be a single commit.
• Logic: I have visually inspected the entire pull request myself.
• Pre-commit checks: All the pre-commits checks have passed.

[forsyth2]

Still in progress. Remaining action items:

• Add assertions that certain lines appear/don't appear in the logs
• Make sure https://app.globus.org/activity shows successful transfers
• Make sure the BLOCKING and NON-BLOCKING cases work