[YUNIKORN-2531] Create unit tests for AsyncRMCallback

[pbacsko]

What is this PR for?

AsyncRMCallback has absolutely no unit test coverage, it's tested only indirectly. This PR adds unit tests to ensure 100% line coverage.

What type of PR is it?

• [ ] - Bug Fix
• [x] - Improvement
• [ ] - Feature
• [ ] - Documentation
• [ ] - Hot Fix
• [ ] - Refactoring

Todos

• [ ] - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/YUNIKORN-2531

How should this be tested?

Screenshots (if appropriate)

Questions:

• [ ] - The licenses files need update.
• [ ] - There is breaking changes for older versions.
• [ ] - It needs documentation.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.47%. Comparing base (1e4bcc0) to head (d10b0f4).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #832      +/-   ##
==========================================
+ Coverage   66.04%   69.47%   +3.42%     
==========================================
  Files          69       69              
  Lines        7537     7531       -6     
==========================================
+ Hits         4978     5232     +254     
+ Misses       2347     2083     -264     
- Partials      212      216       +4     

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[chia7712]

@pbacsko Could you please fix conflicts?