Drop cfg(async_payments) from tests

[tnull]

We previously dropped cfg(async_payments) but didn't drop the actual cargo test call from our CI. Here we do that.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.78%. Comparing base (05f2848) to head (2d7a53f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4165   +/-   ##
=======================================
  Coverage   88.78%   88.78%           
=======================================
  Files         180      180           
  Lines      137004   137004           
  Branches   137004   137004           
=======================================
  Hits       121642   121642           
- Misses      12538    12546    +8     
+ Partials     2824     2816    -8     

Flag	Coverage Δ
fuzzing	20.95% <ø> (ø)
tests	88.63% <ø> (ø)

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[tnull]

Simple enough. CI failures are unrelated.