Add a tool for running the integration tests in a reproducible environment

[exarkun]

Currently the only tool included with the project for creating an execution environment for the integration test suite is tox. This environment is not reproducible since it just depends on virtualenv and pip to satisfy the Python dependencies and inherits all non-Python dependencies from its own execution environment.

Reproducibility for the integration test runtime environment would be a useful step towards reproducibility for the integration test results themselves.

We can leverage the Nix packaging to create such an environment with minimal incremental cost.

[btlogy]

Possibly overlapping ticket:2057 (at least, this one holds some good inputs).

[blaisep]

Investigate ​https://nox.thea.codes/en/stable/tutorial.html

[blaisep]

Investigate ​https://hynek.me/articles/why-i-like-nox/

[blaisep]

Taking over this issues because it impacts new developers.

[blaisep]

This should probably fix ​https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3951