[bbr test] add unit-test to body to header plugin

[nirrozenbaum]

bbr is changing towards pluggable framework. as part of that work a body field to header plugin was added:
https://github.com/kubernetes-sigs/gateway-api-inference-extension/blob/main/pkg/bbr/plugins/body_field_to_header.go

need to add unit-test for the plugin.

cc: @asaadbalum

[asaadbalum]

/assign