Added test for change subscription business plan forcefully

[thisaltennakoon]

$subject

Fix wso2/api-manager#2923
Backend PR: wso2/carbon-apimgt#12467

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 11.55%. Comparing base (b9488e9) to head (dcdbd5b).
Report is 21 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #13512      +/-   ##
============================================
- Coverage     17.43%   11.55%   -5.88%     
+ Complexity      872      593     -279     
============================================
  Files           255      255              
  Lines         11518    11526       +8     
  Branches       1201     1202       +1     
============================================
- Hits           2008     1332     -676     
- Misses         9493    10178     +685     
+ Partials         17       16       -1     

Flag	Coverage Δ
integration_tests	11.55% <ø> (-5.88%)	⬇️

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