Add a simple test for create-sibling-webdav's recursive mode

[adswa]

This fixes #32 with a simple extension to the check_common_workflow function for a recursive invocation of create-sibling-webdav, a check for the appropriate number of results, and a check for the existence of the target directories of the recursively published subdataset.

I debated whether I should create a dedicated test to not inflate the check_common_workflow function, and I narrowly decided against it: By adding it to check_common_workflow the recursive mode gets tested for every mode automatically, and the increase in time is 1.5-2 seconds over the entire test suite. In comparison, a dedicated test function adds 3-4 seconds for just setting up the dataset hierarchy (see timings below). But I'm happy to do it in a dedicated test if others prefer that.

# timing of the test suite without this test:
pytest -s -vv datalad_next/commands/tests/test_create_sibling_webdav.py  14.28s user 7.69s system 59% cpu 36.830 total
# timing of the test suite with the test as in this PR
(fdm-werkstatt) adina@muninn in ~/repos/datalad-next on git:main
pytest -s -vv datalad_next/commands/tests/test_create_sibling_webdav.py  15.72s user 8.99s system 63% cpu 38.869 total
# timing of the testsuite with a dedicated test function for recursive mode (just dataset setup)
pytest -s -vv datalad_next/commands/tests/test_create_sibling_webdav.py  15.55s user 11.98s system 67% cpu 40.996 total

This PR also adds a info about which packages are missing when some webdav tests are skipped.

[codecov]

Codecov Report

Patch coverage: 90.00% and project coverage change: +1.34 🎉

Comparison is base (0cb44b0) 90.70% compared to head (084bd3a) 92.05%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #434      +/-   ##
==========================================
+ Coverage   90.70%   92.05%   +1.34%     
==========================================
  Files         122      122              
  Lines        9100     9109       +9     
==========================================
+ Hits         8254     8385     +131     
+ Misses        846      724     -122     

Impacted Files	Coverage Δ
datalad_next/tests/utils.py	76.72% <0.00%> (ø)
..._next/commands/tests/test_create_sibling_webdav.py	100.00% <100.00%> (ø)

... and 9 files with indirect coverage changes

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[mih]

Thx much and in particular for the great description!