Cabal: Honour -working-dir flag when running test executables

[mpickering]

A simple fix to run the test executable in the -working-dir.

Fixes #10704

Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[sheaf]

Excellent, thanks @mpickering. Is there a similar bug in the code for running benchmarks? IIRC that code is very similar.

[mpickering]

Benchmarks look to be fine. I'll extend the test anyway.

[ulysses4ever]

Thanks!

[ulysses4ever]

Needs only a merge label now after two reviews.

[Mikolaj]

@mpickering: would you like to set the merge label? it's a way of saying you don't want to add anything else to the PR (label needs-review only says it's ready enough to be reviewed).