usage: Rename Opam field "build-test"
usage: Rename Opam field "build-test"

Opam's docs suggest that the `build-test` field is deprecated: https://opam.ocaml.org/doc/Manual.html#opamfield-build-test

Signed-off-by: Kevin Ji <ffb4761cba839470133bee36aeb139f58d7dbaa9@kevinji.com>

janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder,dra27/jbuilder