Use a temp directory for input and output files

[richafrank]

Use a temp directory for input and output files when testing sync, so the working directory remains clean.

As per discussion in #1802 (comment)

I considered changing each test to pass to cli an explicit path to a requirements file in a temp dir, but the autouse fixture seemed more robust to adding new tests that might forget and also continues to test the default reqs file.

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[atugushev]

Awesome! Thanks 🎉