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

[eva-mueller]

When implementing #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

• 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
• 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]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.24%. Comparing base (3b45d85) to head (50ffb8f).
⚠️ Report is 58 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 ⚠️

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     

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[michael-doubez]

LGTM