The color option is not passed to tests

[alygin]

When running cargo test, the --color option affects cargo's output but is not propagated to tests themselves, which prevents coloring their results (ok or FAILED) under certain circumstances when --color=always is used.

The --color option should be passed to tests the same way it's passed to the compiler and other tools.

[matklad]

The same underline issue as #1983. We can't just pass --color=always to test runner because theoretically you can plug in a custom test runner which does not accept this flag.

[alygin]

Thanks! That sounds reasonable.