running cargo test when a rebuild is needed causes mixed text output

[codyps]

This results in (due to rustc emitting warnings) the bolding and coloring of lines to be wrong (some parts that wanted to be yellow are bold & yellow, for example) and some of the warning lines begin at the end of others, or are otherwise mixed together.

cargo should probably be buffering rustc output and emitting rustc output from different invocations separately. I believe ninja is a build system that has already solved this in a similar manner, perhaps it can provide some impl hints.

[alexcrichton]

This is a dupe of #1198 (also reported as #1854), and I think the best course of action here is actually rust-lang/rust#32486, but unfortunately it's impossible to fix the problem everywhere on windows :(