airframe-rx: More test coverage

[xerial]

For #1687

[codecov]

Codecov Report

Merging #1704 (dcc0002) into master (ae94724) will increase coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1704      +/-   ##
==========================================
+ Coverage   82.88%   82.98%   +0.10%     
==========================================
  Files         309      309              
  Lines       11924    11924              
  Branches      766      766              
==========================================
+ Hits         9883     9895      +12     
+ Misses       2041     2029      -12     

Impacted Files	Coverage Δ
...rx/src/main/scala/wvlet/airframe/rx/RxRunner.scala	94.73% <ø> (+3.50%)	⬆️
...frame-rx/src/main/scala/wvlet/airframe/rx/Rx.scala	83.95% <0.00%> (+3.70%)	⬆️
...pec/src/main/scala/wvlet/airspec/spi/Asserts.scala	76.19% <0.00%> (+4.76%)	⬆️

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