put both tests into a single executables stanza to avoid errors

[hannesm]

should solve moby#646 (comment) (at least locally with dune build @install @check @runtest)

[hannesm]

The bin-image failure here is that it didn't update the opam-repository that is embedded in some image. And here I really don't know where and how to fix it, but I'm sure Docker people will know how the container dependencies are laying out.

[patricoferris]

Cheers!