cargo-miri: incorrect behavior when --target is given multiple times

[RalfJung]

When --target is given multiple times, Miri thinks we are building the first target, but actually cargo will build the last target.

[bjorn3]

If you specify --target multiple times, cargo will actually build for every specifies target at the same time, except when doing cargo run where passing multiple --target errors out.

[RalfJung]

Ah true, cargo test actually runs the tests multiple times.

So cargo miri test should support that as well then.