[YUNIKORN-2851] Improve placeholder & placeholder_manger funtion's test coverage

[SP12893678]

What is this PR for?

Improver the following funtion's test coverage:

• newPlaceholder (schedParamsDef not empty case)
• cleanUp (delete pod error case)
• cleanOrphanPlaceholders (delete pod error case)

What type of PR is it?

• - Test

What is the Jira issue?

https://issues.apache.org/jira/browse/YUNIKORN-2851

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.20%. Comparing base (f8e9c11) to head (fdbe5c1).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #905      +/-   ##
==========================================
+ Coverage   68.04%   68.20%   +0.15%     
==========================================
  Files          70       70              
  Lines        7558     7558              
==========================================
+ Hits         5143     5155      +12     
+ Misses       2205     2197       -8     
+ Partials      210      206       -4     

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[craigcondit]

+1 LGTM.