tests(conformance): use router flavor instead of separate flag for enabling expressions router

[pmalek]

What this PR does / why we need it:

Use the same env as elsewhere for router flavor ( TEST_KONG_ROUTER_FLAVOR ) instead of a dedicated one which indicated whether those are enabled or not ( KONG_TEST_EXPRESSION_ROUTES ).

Also add ldflags to fix compilation warnings on mac similarly as it was done in #5829.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.0%. Comparing base (5267051) to head (76a66bf).
Report is 1 commits behind head on main.

❗ Current head 76a66bf differs from pull request most recent head 2d1f17c. Consider uploading reports for the commit 2d1f17c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5860     +/-   ##
=======================================
- Coverage   72.0%   72.0%   -0.1%     
=======================================
  Files        178     178             
  Lines      18210   18210             
=======================================
- Hits       13129   13116     -13     
- Misses      4104    4114     +10     
- Partials     977     980      +3     

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