When a downstream project just uses libcni, it takes a lot of harness
code to set up binaries on-disk that get called.  Instead we can handle
that by passing an execer interface to libcni APIs that gets called
instead, which the downstreams can use to avoid execing thing entirely
and instead check arguments/environment in code.

Downstream users that currently create a CNIConfig object like this:

    cniconf := libcni.CNIConfig{Path: []string{"foobar"}}

Can now fake out all on-disk binary exec/find operations with:

    cniconf := libcni.NewCNIConfig([]string{"foobar"}, <exec interface>)