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

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[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).

[andreasabel]

LGTM.