tests: fix getOutput on stubs

[ncfavier]

Currently, stub packages can't be used with getExe, see #3291 (comment).

Setting outputSpecified prevents getOutput from descending into outputs, which don't have an overridden outPath.

Alternatives:

• removing all outputs makes things fail
• also overriding outPath in outputs is more code

Checklist

• Change is backwards compatible.

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[ncfavier]

Another fix was necessary on top of that because the i3 tests have a stub for i3 but still expect it to be used in a derivation here. This used to work by accident because out was different from the base derivation.

[rycee]

Looks good to me! Merged to master now.