libsubprocess/test: Fix racy fd count in test

[chu11]

Problem: A file descriptor counting test accesses /proc/PID/fd to count file descriptors. The ls used in the test may lead to an additional file descriptor that is not accounted for.

Increase the expected number of file descriptors in the test to account for the one used with ls.

Fixes #6551

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side note: it's possible my guess is wrong. But per some debug output I saw

# stdout: lrwx------ 1 achu achu 64 Jan 13 12:16 0 -> socket:[80034010]
# stdout: lrwx------ 1 achu achu 64 Jan 13 12:16 1 -> socket:[80034012]
# stdout: lrwx------ 1 achu achu 64 Jan 13 12:16 2 -> socket:[80034014]
# stdout: lr-x------ 1 achu achu 64 Jan 13 12:16 3 -> /proc/1128034/fd

I do have evidence that it can appear.

[grondo]

This seems reasonable. Thanks for running it down!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.41%. Comparing base (2bbe885) to head (7018f49).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6553       +/-   ##
===========================================
+ Coverage   41.41%   79.41%   +37.99%     
===========================================
  Files         478      531       +53     
  Lines       79788    88196     +8408     
===========================================
+ Hits        33048    70044    +36996     
+ Misses      46740    18152    -28588     

see 476 files with indirect coverage changes