tests: remove test certs for client_auth tests

[djc]

Forgot these in #343.

[ctz]

Maybe we should move the remaining uses of these files from custom_ekus.rs into client_auth.rs?

edit: and then drop the remaining single test in custom_ekus.rs into integration.rs?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.76%. Comparing base (7badc0e) to head (9847aea).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #344   +/-   ##
=======================================
  Coverage   97.76%   97.76%           
=======================================
  Files          20       20           
  Lines        4348     4348           
=======================================
  Hits         4251     4251           
  Misses         97       97           

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[djc]

Maybe we should move the remaining uses of these files from custom_ekus.rs into client_auth.rs?

edit: and then drop the remaining single test in custom_ekus.rs into integration.rs?

Went a slightly different route? Will probably do more work on this soon.