[Backport 2.x] REST API tests refactoring (#4252 and #4255)

[willyborankin]

Manual backport #4252 #4255 to 2.x.
The failure reason for both was deleted tests.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.94%. Comparing base (4ac861e) to head (512a112).

Additional details and impacted files

@@            Coverage Diff             @@
##              2.x    #4328      +/-   ##
==========================================
- Coverage   66.05%   65.94%   -0.11%     
==========================================
  Files         301      301              
  Lines       21581    21581              
  Branches     3513     3513              
==========================================
- Hits        14255    14232      -23     
- Misses       5569     5588      +19     
- Partials     1757     1761       +4     

Files	Coverage Δ
...arch/security/ssl/OpenSearchSecuritySSLPlugin.java	86.30% <ø> (ø)

... and 10 files with indirect coverage changes