ci: wire standalone test suite into GitHub Actions

What happened

The standalone test suite under test/standalone is guarded by the standalone build tag, but it does not appear to be wired into the GitHub Actions CI workflow or any Make target used by CI.

Why this matters

Host/standalone mode has process-level coverage in test/standalone, but because the suite is not run by CI, regressions in standalone mode can slip through. This also makes it harder to maintain confidence in non-Kubernetes infrastructure-provider behavior over time.

Suggested fix

Add a CI target for the standalone suite, for example:

cd test && go test -v -tags standalone ./standalone

If needed, CI can prebuild the envoy-gateway binary and pass it through ENVOY_GATEWAY_BIN before running the standalone suite.

Expected behavior

The standalone test suite should run reliably in GitHub Actions.