airspec: #2163 Support async test for Rx[_]

[xerial]

Closes #2163

[codecov]

Codecov Report

Merging #2652 (dc3d6dd) into master (6b766a6) will decrease coverage by 0.01%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2652      +/-   ##
==========================================
- Coverage   82.12%   82.10%   -0.02%     
==========================================
  Files         334      334              
  Lines       14030    14030              
  Branches     2182     2182              
==========================================
- Hits        11522    11520       -2     
- Misses       2508     2510       +2     

Impacted Files	Coverage Δ
...rx/src/main/scala/wvlet/airframe/rx/RxRunner.scala	94.09% <0.00%> (-0.79%)	⬇️

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