Define public API of datalad_next.tests

[mih]

This pretty much takes the role of datalad_next.tests.utils. The new setup treats tests like any other sub-package and exposes reusable tools as a public API at the top-level.

In doing so, SkipTest was made obsolete and excluded from that API.

The usage of rmtree is altered to import from datalad_next.utils.

#613

[codecov]

Codecov Report

Attention: 32 lines in your changes are missing coverage. Please review.

Comparison is base (7e79ccf) 15.97% compared to head (902efea) 21.43%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
datalad_next/tests/fixtures.py	0.00%	7 Missing ⚠️
...d_next/patches/tests/test_push_to_export_remote.py	0.00%	3 Missing ⚠️
...atalad_next/gitremotes/tests/test_datalad_annex.py	0.00%	2 Missing ⚠️
...atalad_next/iter_collections/tests/test_iterdir.py	0.00%	2 Missing ⚠️
datalad_next/tests/__init__.py	0.00%	2 Missing ⚠️
datalad_next/annexbackends/tests/test_base.py	0.00%	1 Missing ⚠️
datalad_next/annexremotes/tests/test_archivist.py	0.00%	1 Missing ⚠️
datalad_next/annexremotes/tests/test_uncurl.py	0.00%	1 Missing ⚠️
..._next/commands/tests/test_create_sibling_webdav.py	0.00%	1 Missing ⚠️
datalad_next/commands/tests/test_download.py	0.00%	1 Missing ⚠️
... and 11 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #619      +/-   ##
==========================================
+ Coverage   15.97%   21.43%   +5.45%     
==========================================
  Files         164      165       +1     
  Lines       11499    11502       +3     
  Branches     1345     1345              
==========================================
+ Hits         1837     2465     +628     
+ Misses       9622     8977     -645     
- Partials       40       60      +20     

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