Add missing conf-npm to alcotest-js.1.9.1

[jmid]

This PR adds a missing conf-npm to alcotest-js.1.9.1 to avoid Program node not found in the tree or in PATH.

I can see @mseri added the same line for 1.9.0 in #27617. It would probably be good to upstream.

This was missed in #28629 and I see 72(!) occurrences of

alcotest-js.1.9.1
  tests (failed: Program node not found in the tree or in PATH)

in the still live https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/9ccf5c243215d84a1b06e9750cb040d14252b29d
from 6 days ago, so it seems like a premature merge @smorimoto 😉

I just spotted the error on #28662 where it showed up again.

[jmid]

This is down to

• opam-2.0 failures (complaining about ppxlib.0.36.2 which hasn't been touched) and
• topkg.1.1.0 failing to compile on MSys2 MSys2 fails to compile topkg.1.1.0  #28636

This should thus be good to go.

I've also upstreamed the missing test dependency here: mirage/alcotest#427.

[mseri]

Thanks