Update Pause test code

[moglu2017]

Description

Update staker_native_pause_test.go.
Remove staker.ActivateNextValidator function because it will chanage to private.

Type of change

• Update test case

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.22%. Comparing base (e88bbb8) to head (2aff8fa).
⚠️ Report is 2 commits behind head on release/hayabusa.

Additional details and impacted files

@@                 Coverage Diff                  @@
##           release/hayabusa    #1220      +/-   ##
====================================================
- Coverage             58.23%   58.22%   -0.02%     
====================================================
  Files                   273      273              
  Lines                 28961    28961              
====================================================
- Hits                  16866    16863       -3     
- Misses                10628    10630       +2     
- Partials               1467     1468       +1     

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