Allow to override block_on function

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 18 lines in your changes missing coverage. Please review.

Project coverage is 90.06%. Comparing base (d87d9b2) to head (f1577b8).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
ntex-rt/src/builder.rs	88.00%	9 Missing ⚠️
ntex-rt/src/system.rs	83.78%	6 Missing ⚠️
ntex-rt/src/arbiter.rs	62.50%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #411   +/-   ##
=======================================
  Coverage   90.05%   90.06%           
=======================================
  Files         202      202           
  Lines       32378    32446   +68     
=======================================
+ Hits        29157    29221   +64     
- Misses       3221     3225    +4     

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