test: add improved preauth tests

[aeneasr]

Related issue

Proposed changes

Checklist

• I have read the contributing guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further comments

[codecov]

Codecov Report

Merging #758 (f03f28d) into master (57f62d5) will decrease coverage by 0.02%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master     #758      +/-   ##
==========================================
- Coverage   61.90%   61.88%   -0.03%     
==========================================
  Files         102      102              
  Lines        4024     4027       +3     
==========================================
+ Hits         2491     2492       +1     
- Misses       1266     1267       +1     
- Partials      267      268       +1     

Impacted Files	Coverage Δ
...peline/authn/authenticator_oauth2_introspection.go	76.82% <42.85%> (-0.89%)	⬇️

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