Allow testing optional dependencies

Allow testing optional dependencies

Previously a warning was issued if both -p and --features were passed as flags,
and the rationale for this was that if --features modified the activated set of
features in the package selected by -p it would alter Cargo.lock, which is
undesirable as Cargo.lock should be stable.

This commit, however, interprets --features as changing the resolved graph of
the top-level package, and then -p is a query on that resolved graph. This way
the Cargo.lock file never changes and you're allowed to test optional
dependencies.