Adjust test to find xclim location via importlib

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR addresses an issue raised in xclim v0.54.0 conda-forge/xclim-feedstock#89
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• The TestOfficialYaml test now uses a dynamic method for locating the installed location of xclim.

Does this PR introduce a breaking change?

No.

Other information:

When xclim is installed, there is no guarantee that src will be the top-level folder. This is by design to ensure that we don't write tests that try to navigate the folder-tree of xclim using relative paths from the tests folder. Seeing as this is the only failing test on conda-forge, I think we're doing an OK job.

Commit that bypasses this problem on conda-forge: conda-forge/xclim-feedstock@f1886ee