Avoid resource warning false positive in unit test

[dimbleby]

I've been getting ResourceWarning for unclosed files from a handful of unit tests. The first commit in this MR wasn't the right guess, I eventually tracked it down to the second.

This looks like it's probably a bug somewhere in pytest I suppose, maybe some unexpected interaction between fixtures and capturing warnings.

This change strictly speaking makes the test less powerful - in the unlikely event that someone were to remove the deprecation warning the unit test wouldn't notice any more. But IMO it's better to live with that than with the false positives.

With any luck this all goes away soon anyway when #7356 completes the deprecation

before:

$ pytest -k test_add_no_constraint_old_installer
===================================================== test session starts =====================================================
platform linux -- Python 3.10.6, pytest-7.2.1, pluggy-1.0.0
Using --randomly-seed=335022381
rootdir: /home/dch/poetry, configfile: pyproject.toml, testpaths: tests
plugins: mock-3.10.0, xdist-3.2.0, cov-4.0.0, randomly-3.12.0
gw0 [1] / gw1 [1] / gw2 [1] / gw3 [1]
.                                                                                                                       [100%]
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=0 mode='r' encoding='UTF-8'>
sys:1: ResourceWarning: unclosed file <_io.TextIOWrapper name=1 mode='w' encoding='UTF-8'>

====================================================== 1 passed in 2.83s ======================================================

after

$ pytest -k test_add_no_constraint_old_installer
===================================================== test session starts =====================================================
platform linux -- Python 3.10.6, pytest-7.2.1, pluggy-1.0.0
Using --randomly-seed=3521203966
rootdir: /home/dch/poetry, configfile: pyproject.toml, testpaths: tests
plugins: mock-3.10.0, xdist-3.2.0, cov-4.0.0, randomly-3.12.0
gw0 [1] / gw1 [1] / gw2 [1] / gw3 [1]
.                                                                                                                       [100%]
====================================================== 1 passed in 2.01s ======================================================

[github-actions]

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