Use TLS instead of set_boundary() for multipart assertions

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

This is a hack until reqwest merges seanmonstar/reqwest#2814

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

There could potentially be concurrency bugs. Since it's using TLS, it would have to be bugs with multiple tests running concurrently on a single thread. As far as I know, tokio doesn't do that. I ran the tests a bunch too to test. It could also fuck up if multiple bodies are rendered concurrently in one test, but we don't do that either.

QA

How did you test this?

Ran the tests 100 times.

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked