CLN: Remove testdata logic from userspace

[mferrera]

Resolves #1093 Resolves #1176

This is a large PR but the changes it makes are uniform and simple:

• Moved top-level conftest.py into tests/conftest.py
• Removed testdata path stuff from XTGeoDialog
• Removed how that was previously accessed in most test files
• Switched it to be accessed through a pytest fixture
• Moved all usage of pytest's tmpdir to tmp_path

This also means that XTG_TESTPATH or --testdatapath are fully honored by pytest now.

Grouped this last point in with the rest to make it a single pass over every test rather than two passes.

Ran this in the RMS environment on-prem and it is fine there too.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (0314127) 80.01% compared to head (9cae908) 80.01%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1177 +/- ## ========================================== - Coverage 80.01% 80.01% -0.01% ========================================== Files 98 98 Lines 13700 13679 -21 Branches 2199 2197 -2 ========================================== - Hits 10962 10945 -17 + Misses 2006 2002 -4 Partials 732 732 ```

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