Refactor UnusedPrivateMemberSpec

[chao2zhang]

This is a preparation PR for #3418 to

• Refactor off UnusedPrivateMemberNegative.kt and UnusedPrivateMemberPositive.kt
• Extracted tests related to parameters into UnusedPrivateParameterSpec.kt

With smaller modularized unit tests, it would be easy to review future PRs.

[codecov]

Codecov Report

Merging #3667 (8964043) into main (2c4b6fb) will not change coverage.
The diff coverage is n/a.

❗ Current head 8964043 differs from pull request most recent head a183598. Consider uploading reports for the commit a183598 to get more accurate results

@@            Coverage Diff            @@
##               main    #3667   +/-   ##
=========================================
  Coverage     77.95%   77.95%           
  Complexity     2870     2870           
=========================================
  Files           470      470           
  Lines          9245     9245           
  Branches       1761     1761           
=========================================
  Hits           7207     7207           
  Misses         1077     1077           
  Partials        961      961           

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