Add integration tests

[frostming]

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (add-tests@3ed2038). Click here to learn what that means. The diff coverage is 48.89%.

@@             Coverage Diff              @@
##             add-tests      #66   +/-   ##
============================================
  Coverage             ?   54.72%           
============================================
  Files                ?       42           
  Lines                ?     2701           
  Branches             ?      475           
============================================
  Hits                 ?     1478           
  Misses               ?     1108           
  Partials             ?      115

Impacted Files	Coverage Δ
src/passa/cli/install.py	0% <ø> (ø)
src/passa/cli/options.py	78.43% <ø> (ø)
src/passa/cli/add.py	0% <ø> (ø)
src/passa/internals/_pip_shims.py	72.97% <ø> (ø)
src/passa/cli/remove.py	0% <ø> (ø)
src/passa/actions/sync.py	0% <ø> (ø)
src/passa/cli/upgrade.py	0% <ø> (ø)
src/passa/models/environments.py	22.67% <0%> (ø)
src/passa/models/providers.py	61.29% <0%> (ø)
src/passa/internals/utils.py	64.91% <0%> (ø)
... and 12 more

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[frostming]

Creating virtualenv for every test case is a big performance killer in testing. I am considering to design a new (un)installation interface so that we can get a full mocked (un)installation and get rid of venv.

[frostming]

Whoa, after mocking installation, the time cost is reduced by 60%!

[frostming]

I am not apt to make huge PRs, so if the review is done we can start merging stuff. No need to have a full test suite at this point. Especially there are some bugfixes that are worth merging into the master. Do we need to cherry-pick?