DM-44109: Protect tests against httpx being available but no server deps

[timj]

776aaea changed the test assumptions. CernVMFS installations now do include httpx but do not include anything the server needs for testing.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes
• (if changing dimensions.yaml) make a copy of dimensions.yaml in configs/old_dimensions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.05%. Comparing base (f490737) to head (9ecd191).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1007   +/-   ##
=======================================
  Coverage   89.05%   89.05%           
=======================================
  Files         347      347           
  Lines       44507    44507           
  Branches     9149     9149           
=======================================
+ Hits        39635    39636    +1     
  Misses       3550     3550           
+ Partials     1322     1321    -1     

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