tests: fix environment not being replaced soon enough

[onerandomusername]

continuation of #120

That solution using a fixture did not patch the environment soon enough. This will patch the environment and save a cache of the original environment, so it can replaced upon the end of a test run.

[codecov]

Codecov Report

Merging #126 (67d19f5) into main (4b07917) will decrease coverage by 0.38%.
The diff coverage is 47.36%.

❗ Current head 67d19f5 differs from pull request most recent head f503ae6. Consider uploading reports for the commit f503ae6 to get more accurate results

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
- Coverage   70.90%   70.52%   -0.39%     
==========================================
  Files          33       33              
  Lines        1619     1632      +13     
  Branches      199      203       +4     
==========================================
+ Hits         1148     1151       +3     
- Misses        439      447       +8     
- Partials       32       34       +2     

Impacted Files	Coverage Δ
tests/modmail/conftest.py	63.88% <47.36%> (-23.07%)	⬇️

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[onerandomusername]

After further inspection, this should keep the environment unmodified, only removing all variables prefixed with MODMAIL_ in order to keep the environment more realistic of what would happen on a real environment, and only controlling the program specific environment variables.

[onerandomusername]

However, environment variables are currently not prefixed with MODMAIL_, as that is introduced in #75.

[onerandomusername]

This can be reviewed and merged, but will only have an effect on the testing environment once GH-75 is merged

[onerandomusername]

Merging main to get the updated workflows here.

Will rebase before merge.

[coveralls]

Coverage decreased (-0.05%) to 72.981% when pulling f503ae6 on tests/fix-environment into 8ba34ba on main.

[Shivansh-007]

This use modmail.config.ENV_PREFIX once config PR is merged, worth adding a TODO