Convert TestApp fns to async/await

[Turbo87]

The creation of users and tokens is currently hard to convert to diesel-async because the test suite is using sync fn calls to create them. This PR migrates the fns to async/await, even if internally some of them are not actually using async/await yet. This makes it much easier to port these to diesel-async in follow-up pull requests.

[codecov]

Codecov Report

Attention: Patch coverage is 99.66443% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.14%. Comparing base (1032ce9) to head (8327b3d).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
src/tests/util/test_app.rs	92.85%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9987      +/-   ##
==========================================
- Coverage   89.20%   89.14%   -0.07%     
==========================================
  Files         292      294       +2     
  Lines       30412    30590     +178     
==========================================
+ Hits        27130    27269     +139     
- Misses       3282     3321      +39     

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