test: cram coverage for the three new gen options Add 4 new cases to test/rules.t/run.t: - --internal-prefix forwards the option to ocsigen-ppx-client. - --subdir wraps every rule in (subdir DIR ...). - --server-objs-dir without --subdir generates explicit %{dep:DIR/Name.cmo} paths. - --server-objs-dir combined with --subdir derives the wrapping prefix from the subdir name, e.g. [Os] -> [os__Name.cmo]. Each new test re-runs gen on the existing test fixture (a.eliom, b.eliom, b.eliomi) so the diff between the default invocation and the option-enabled invocations is easy to read.