Add test that checks whether client can find at least one base URL

[ml-evs]

This has potential to be flaky if the providers list is down, but think that is a slim chance and this would have caught the previous issue in #1848.

[codecov]

Codecov Report

Merging #1849 (edf65c8) into master (2970d9a) will increase coverage by 0.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1849      +/-   ##
==========================================
+ Coverage   90.53%   91.22%   +0.69%     
==========================================
  Files          75       75              
  Lines        4627     4629       +2     
==========================================
+ Hits         4189     4223      +34     
+ Misses        438      406      -32     

Flag	Coverage Δ
project	91.22% <100.00%> (+0.69%)	⬆️
validator	90.68% <100.00%> (+0.69%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
optimade/client/client.py	87.14% <ø> (-0.05%)	⬇️
optimade/utils.py	91.20% <100.00%> (+35.52%)	⬆️

... and 1 file with indirect coverage changes