rustpkg test claims to support --cfg but doesn't

[sfackler]

 ❯ rustpkg -c travis test                                                                                                           [rust-postgres/master=]
WARNING: The Rust package manager is experimental and may be unstable
The --cfg option can only be used with the build or install commands.
rustpkg [options..] test

Build all test crates in the current directory with the test flag.
Then, run all the resulting test executables, redirecting the output
and exit code.

Options:
    -c, --cfg      Pass a cfg flag to the package script

[sfackler]

cc @catamorphism