chore(tests): rename TestUDPRoute to TestUDPRouteEssentials and make similar to TestTCPRouteEssentials

[programmer04]

What this PR does / why we need it:

Tests for UDP in KIC are historically overall complex, they use CoreDNS for verification. This PR renames TestUDPRoute toTestUDPRouteEssentials and makes it much simpler (uses the same approach as TestTCPRouteEssentials). It also tests more features (read more in the next section).

Which issue this PR fixes:

• Part of the efforts to fight flakiness Flaky test TestExampleUDPRoute #5715.
• Simplifying tests and getting rid of CoreDNS used in tests.
• Refactoring promised in fix: set proper destination port for TCPRoute and UDPRoute #4928 to cover introduced improvements for UDPRoute with proper test too

Special notes for your reviewer:

Some small opportunistic improvements to TestTCPRouteEssentials have been made too. The last occurrence of CoreDNS in test/integration/udpingress_test.go will be handled in a separate PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.8%. Comparing base (8c49b29) to head (c03adf3).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5723   +/-   ##
=====================================
  Coverage   71.7%   71.8%           
=====================================
  Files        175     175           
  Lines      18141   18141           
=====================================
+ Hits       13019   13031   +12     
+ Misses      4170    4165    -5     
+ Partials     952     945    -7     

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