refactor(nextcloud_test): unify test instance handling

[Leptopoda]

Why?

1. this cleans up the tests themselves, as they no longer need to handle the client creation and target lifecycle.
2. The tests can be split into multiple files (maybe per sub client) for better structure

[Leptopoda]

Making the test factory a singleton is not strictly needed, as the NextcloudTester could handle the creation and lifecycle.
But IMO there is no need for running tests against two backends (docker and local) at the same time.

[provokateurin]

Very nice cleanups, thank you 👍

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.85%. Comparing base (07d1eba) to head (9cffb7d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2325   +/-   ##
=======================================
  Coverage   29.85%   29.85%           
=======================================
  Files         293      293           
  Lines      117895   117895           
=======================================
  Hits        35196    35196           
  Misses      82699    82699           

Flag	Coverage Δ		*Carryforward flag
cookie_store	100.00% <ø> (ø)		Carriedforward from 07d1eba
dynamite	31.08% <ø> (ø)		Carriedforward from 07d1eba
dynamite_end_to_end_test	60.96% <ø> (ø)		Carriedforward from 07d1eba
dynamite_runtime	85.40% <ø> (ø)		Carriedforward from 07d1eba
neon_dashboard	96.20% <ø> (ø)
neon_framework	45.85% <ø> (ø)
neon_http_client	93.28% <ø> (ø)
neon_notifications	100.00% <ø> (ø)
neon_talk	99.46% <ø> (ø)
nextcloud	26.76% <ø> (ø)
sort_box	90.90% <ø> (ø)		Carriedforward from 07d1eba

*This pull request uses carry forward flags. Click here to find out more.