chore: validate type assertion to prevent panic

[reversearrow]

I observed panics couple of times in tests. Validating type assertion before proceeding further should prevent this panic.

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Codecov Report

Attention: Patch coverage is 33.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 59.42%. Comparing base (4a817ad) to head (0029337).

Files	Patch %	Lines
kong/test_utils.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
- Coverage   59.44%   59.42%   -0.03%     
==========================================
  Files          71       71              
  Lines        4392     4394       +2     
==========================================
  Hits         2611     2611              
- Misses       1170     1171       +1     
- Partials      611      612       +1     

Flag	Coverage Δ
2.1	35.75% <0.00%> (-0.02%)	⬇️
2.2	48.31% <33.33%> (-0.03%)	⬇️
2.3	48.95% <33.33%> (-0.03%)	⬇️
2.4	48.99% <33.33%> (-0.03%)	⬇️
2.5	48.99% <33.33%> (-0.03%)	⬇️
2.6	48.99% <33.33%> (-0.03%)	⬇️
2.7	50.70% <33.33%> (-0.03%)	⬇️
2.8	50.70% <33.33%> (-0.03%)	⬇️
3.0	54.64% <33.33%> (-0.03%)	⬇️
3.1	56.28% <33.33%> (-0.03%)	⬇️
3.2	56.32% <33.33%> (-0.03%)	⬇️
3.3	56.32% <33.33%> (-0.03%)	⬇️
3.4	58.71% <33.33%> (-0.03%)	⬇️
3.5	56.50% <33.33%> (-0.03%)	⬇️
3.6	56.50% <33.33%> (-0.03%)	⬇️
community	43.42% <0.00%> (-0.02%)	⬇️
enterprise	58.03% <33.33%> (-0.03%)	⬇️
integration	59.42% <33.33%> (-0.03%)	⬇️

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[rainest]

This is presumably caused by the test checking the admin API before it's properly available. We should try to make that not happen, but I suppose this is at least slightly better than the panic.