Fix host_out readlines in test_utils

[YunchuWang]

Description

Fixes #
Add a check to read line from host_out only if stdout is readable in integration test.

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[codecov]

Codecov Report

Merging #976 (1122069) into dev (401801f) will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##              dev     #976      +/-   ##
==========================================
- Coverage   81.04%   81.01%   -0.03%     
==========================================
  Files          35       35              
  Lines        2337     2339       +2     
  Branches      370      371       +1     
==========================================
+ Hits         1894     1895       +1     
  Misses        337      337              
- Partials      106      107       +1     

Flag	Coverage Δ
unittests	80.97% <66.66%> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
azure_functions_worker/testutils.py	73.43% <66.66%> (-0.09%)	⬇️

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