Re-enable tests of bash completion functions for gz

[mjcarroll]

Follow-up from #478

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (eac2e69) 87.69% compared to head (014e40e) 87.69%.
Report is 2 commits behind head on gz-transport13.

Additional details and impacted files

@@               Coverage Diff               @@
##           gz-transport13     #481   +/-   ##
===============================================
  Coverage           87.69%   87.69%           
===============================================
  Files                  59       59           
  Lines                5704     5704           
===============================================
  Hits                 5002     5002           
  Misses                702      702           

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[caguero]

Thanks!

homebrew looks unhappy

[mjcarroll]

homebrew looks unhappy

I'm not sure these tests worked on homebrew before, I will limit them to just Linux.

[mjcarroll]

Trying something slightly different and starting with /usr/bin/bash and falling back to /bin/bash if that isn't available. The subprocess class doesn't do anything with the PATH environment variable, you have to have the full and complete path to the binary in question.

[mjcarroll]

@scpeters did a brief inspection for me on macOS and things didn't seem to work, likely because of some security mechanism of the operating system. Since the value of this test is mainly in checking the flags for consistency and keeping them in sync between the completion scripts and the executables, running cross-platform isn't really necessary here.