Issue #323: bash engine: drain stdout of instrumented script

[dciabrin]

When bash supports xtrace fd, no kcov marker can come from the instrumented script's stdout, so it's safe to drain it as soon as data is written to it, without waiting for '\n'.

stdout writers: Sum code/executed lines before generating headers

[codecov[bot]]

Codecov Report

Merging #324 into master will increase coverage by 0.10%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #324      +/-   ##
==========================================
+ Coverage   70.18%   70.28%   +0.10%     
==========================================
  Files          57       57              
  Lines        3706     3719      +13     
==========================================
+ Hits         2601     2614      +13     
  Misses       1105     1105              

Impacted Files	Coverage Δ
src/include/utils.hh	77.27% <ø> (ø)
src/engines/bash-engine.cc	89.18% <100.00%> (+0.22%)	:arrow_up:
src/utils.cc	62.40% <100.00%> (+1.04%)	:arrow_up:

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[coveralls]

Coverage increased (+0.1%) to 70.288% when pulling 200952016b9462bd8b73456254a5731eda3704ad on dciabrin:drain-stdout into 64eb351d8e1baa491f6a23bb7ea912b0844ee6fd on SimonKagstrom:master.

[SimonKagstrom]

Thanks a lot for the fix!