Update tests and make authorization header check for `Basic` case-insensitive (RFC 7235)

[eva-mueller-coremedia]

When implementing https://github.com/jenkinsci/oic-auth-plugin/pull/480 I got aware of some things which can be cleaned up like. In the course of this small fix, I tried to understand the integration test in PluginTest.java and found it a little bit ... grown. Thus I made some housekeeping

• move mock methods to own file
• move helper methods to own file
• extracted topic related tests from PluginTest.java into own test files

Testing done

Only covered by unit tests.

Submitter checklist

• [x] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ ] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [ ] Link to relevant issues in GitHub or Jira
• [ ] Link to relevant pull requests, esp. upstream and downstream changes
• [ ] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov[bot]]

Codecov Report

:x: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 76.24%. Comparing base (3b45d85) to head (50ffb8f). :warning: Report is 11 commits behind head on master.

Files with missing lines	Patch %	Lines
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	72.72%	0 Missing and 3 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #679      +/-   ##
============================================
+ Coverage     74.60%   76.24%   +1.63%     
- Complexity      315      325      +10     
============================================
  Files            33       33              
  Lines          1280     1284       +4     
  Branches        176      178       +2     
============================================
+ Hits            955      979      +24     
+ Misses          241      224      -17     
+ Partials         84       81       -3     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• :package: JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.