tests(iam): fix conflicting user test

[estellesoulard]

Nightly tests are failing with the error: 'user with the same username or email found in organization'

• Add unique user attributes (email, username)
• Add missing user destroy check

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 1.77%. Comparing base (8dcb144) to head (08cba76).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/services/iam/testfuncs/checks.go	0.00%	16 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #3483      +/-   ##
=========================================
- Coverage    1.77%   1.77%   -0.01%     
=========================================
  Files         405     405              
  Lines       44273   44287      +14     
=========================================
  Hits          786     786              
- Misses      43400   43414      +14     
  Partials       87      87              

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