Fix tox testing

[noirbizarre]

Description

Tox test suite (poetry test:all) is currently failing:

• installation was KO
• cwd was assuming to the repository in multiple tests while it's not the case in tox:
  • test data file references were root-relative
  • some test commands were only working in the root of repository

This PR fixes the tox test suite with the current stack.

The only remaining failures are those fixed in #1726 (@bearomorphism this might interest and help you testing against all versions)

Checklist

• I have read the contributing guidelines

Code Changes

• Use the testdependency-groups in tox
• Use pytest-gitconfig instead of the git_sandbox fixture (allows safe and easy changes on gitconfig, (cf. test_git which was setting some local git config during the test)
• Introduces 3 new fixtures (and use them when required as toxcwd is not the repository root):
  • repo_root: path to the current repository root
  • in_repo_root: change the current cwd to the repository root
  • data_dir: path to the test data

Documentation Changes

No documentation change

Expected Behavior

poetry test:all works and failure are legit broken cases.

Steps to Test This Pull Request

1. Checkout master or v4-11-0
2. Run poetry install --all-groups
3. Run poetry test:all: it fails to run
4. Checkout this branch
5. Run poetry install --all-groups
6. Run poetry test:all: it runs. Only failure are for 3.13 and 3.14, the cases being fixed by test(commands): centralize all --help tests in a file to dedup code #1726

Additional Context

I think this is the first PR of a "cleaning the tests" serie.
Might come after in other PRs:

• all warning fixes (too much scrolling required to see the test results)
• path consistency: *_path fixtures everywhere, Pathlib-based everywhere, dedup some inner conftest fixtures

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.84%. Comparing base (2072f8e) to head (a2b127a).
⚠️ Report is 3 commits behind head on v4-11-0.

Additional details and impacted files

@@ Coverage Diff @@## v4-11-0 #1740 +/- ##
========================================
Coverage 97.84% 97.84% ========================================
Files 60 60 Lines 2604 2604 ========================================
Hits 2548 2548 Misses 56 56 

Flag	Coverage Δ
unittests	97.84% <ø> (ø)

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