Remove redundant import error
Fix failing test
Don't create subprocesses for the `rust` command

Instead link against the built libraries and directly invoke those.
actually fix failing test
auto merge of #6400 : cmr/rust/remove_useless_import_error, r=thestinger

Every unresolved import is reported. An additional error message isn't useful
and obscures (imo) the real errors: I need to take it into account when
looking at the error count.
auto merge of #6438 : alexcrichton/rust/issue-5387, r=thestinger

Instead link against the built libraries and directly invoke those.

Closes #5387