Document --nocapture to reduce confusion running tests.
Document --nocapture to reduce confusion running tests.

Although this is a feature of rustc's test harness and
not cargo's test driver, I can never remember how to
invoke it. Describing the switch in `cargo test --help`
makes it more discoverable.

durka/cargo,achanda/cargo,eminence/cargo,AndiDog/cargo,brson/cargo,blaenk/cargo,AndiDog/cargo,blaenk/cargo,tsurai/cargo,gchp/cargo,mark-buer/cargo,rustyhorde/cargo,blaenk/cargo,gchp/cargo,rust-lang/cargo,brson/cargo,vojtechkral/cargo,ThinTim/cargo,eminence/cargo,mark-buer/cargo,tshepang/cargo,rust-lang/cargo,eminence/cargo,mark-buer/cargo,nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo,tshepang/cargo,AndiDog/cargo,tshepang/cargo,tsurai/cargo,ThinTim/cargo,vojtechkral/cargo,sbeckeriv/cargo,rustyhorde/cargo,nipunn1313/cargo,gchp/cargo,brson/cargo,brson/cargo,durka/cargo,vojtechkral/cargo,tsurai/cargo,AndiDog/cargo,rustyhorde/cargo,blaenk/cargo,achanda/cargo,sbeckeriv/cargo,AndiDog/cargo,achanda/cargo,ThinTim/cargo,durka/cargo,gchp/cargo,nipunn1313/cargo,durka/cargo,blaenk/cargo,rust-lang/cargo,mark-buer/cargo,sbeckeriv/cargo,ThinTim/cargo,eminence/cargo,tsurai/cargo,brson/cargo,tshepang/cargo,nipunn1313/cargo,vojtechkral/cargo,achanda/cargo,ThinTim/cargo,sbeckeriv/cargo,tsurai/cargo,mark-buer/cargo,gchp/cargo