Test README references Makefile targets that don't exist

[pleshakov]

https://github.com/nginxinc/nginx-gateway-fabric/blob/main/tests/README.md#option-2-install-nginx-gateway-fabric-from-local-already-built-image-to-configured-kind-cluster references Makefile target that no longer exist

make update-ngf-manifest
make update-ngf-manifest-with-plus

Problems was introduced in 18671fb

Acceptance criteria:

• Ensure that the steps in Option 2 Install NGINX Gateway Fabric from local already built image to configured kind cluster can be performed.

[github-actions]

This issue is stale because it has been open 14 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[kate-osborn]

The tests Makefile also references makefile targets that do not exist.

For example, this target: https://github.com/nginxinc/nginx-gateway-fabric/blob/3ca9a69b12b101630fbcac6fbe0baea508a28c6d/tests/Makefile#L174 calls load-images and helm-install-local, which are in the top-level Makefile.

Additionally, this target: https://github.com/nginxinc/nginx-gateway-fabric/blob/3ca9a69b12b101630fbcac6fbe0baea508a28c6d/tests/Makefile#L171 looks incorrect. It seems like it only deploys the updated provisioner

[lucacome]

@kate-osborn the Makefile in tests includes the main Makefile so that we don't have to have duplicated targets