Refactor mock webserver into a fixture

[jonathangreen]

Description

This makes two changes:

• Turn mock_web_server into a fixture, since we use the same fixture function in a few places
• Make sure when we are calling request_with_timeout we set timeout, backoff_factor, so that the tests don't have to wait for retries. This was making tests slower then they need to be.

Motivation and Context

I was troubleshooting what was causing slow tests in #1554. This was not the cause, but it was making out tests take longer then they needed to, so I thought I'd fix it while I noticed since it cuts down the time to run our core tests by about a minute for me.

How Has This Been Tested?

• Running unit tests

Checklist

• I have updated the documentation accordingly.
• All new and existing tests passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a170786) 90.24% compared to head (30c1eb9) 90.24%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1564      +/-   ##
==========================================
- Coverage   90.24%   90.24%   -0.01%     
==========================================
  Files         237      237              
  Lines       28606    28606              
  Branches     6538     6538              
==========================================
- Hits        25816    25815       -1     
  Misses       1837     1837              
- Partials      953      954       +1     

Flag	Coverage Δ
Api	75.34% <ø> (ø)
Core	59.93% <ø> (ø)
migration	26.87% <ø> (ø)

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[tdilauro]

The changes look good to me! 🍾