router_check_tool: Load runtime flags before running the tests

[cainelli]

Commit Message: router_check_tool: Load runtime flags before running the tests. Additional Description: This adds a new field to the Validation configuration to load runtime flags. More context in https://github.com/envoyproxy/envoy/pull/35662#discussion_r1794234910 Risk Level: Low Testing: Extended existing tests. Without re2.max_program_size.error_level flag set the test fails.

bazel-bin/test/tools/router_check/router_check_tool -c test/tools/router_check/test/config/DefaultRuntime.yaml -t test/tools/router_check/test/config/DefaultRuntime.golden.proto.json

Fixes: https://github.com/envoyproxy/envoy/issues/35174

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/36660 was opened by cainelli.

see: more, trace.