Loki: Turn test sub-directories into sub-pacakges

[mlange05]

This make test integration cleaner and avoids eventual namespace clashes in the future. It also now runs the linter over tests, without exporting test symbols as part of from loki import * (please double-check this).

Note, the other half of this is hidden in PR #446 , where this was necessary to avoid namespace clashes in tests.

EDIT: Looks like I have to leave loki.lint.tests untouched, as the introduction of the test sub-packages makes the dynamic rules imports go haywire. I leave that for a follow-on PR.

[github-actions]

Documentation for this branch can be viewed at https://sites.ecmwf.int/docs/loki/447/index.html

[reuterbal]

Many thanks for this, changes look good and the linter-compliance pass on the tests is very much appreciated!

[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.19%. Comparing base (86a0f8d) to head (3e0e8ab).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
loki/build/tests/test_build.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
==========================================
- Coverage   93.33%   93.19%   -0.14%     
==========================================
  Files         218      219       +1     
  Lines       40848    40908      +60     
==========================================
- Hits        38127    38126       -1     
- Misses       2721     2782      +61     

Flag	Coverage Δ
lint_rules	96.39% <ø> (ø)
loki	93.15% <96.42%> (-0.15%)	⬇️

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