Enable colored test output when capturing output of tests

The output of individual tests can be captured now so it's safe to use
colorized output even when running tests in parallel. Closes #782.

src/libtest/lib.rs

@@ -740,7 +740,7 @@ fn should_sort_failures_before_printing_them() {
 
 fn use_color(opts: &TestOpts) -> bool {
     match opts.color {
-        AutoColor => get_concurrency() == 1 && stdout_isatty(),
+        AutoColor => !opts.nocapture && stdout_isatty(),
         AlwaysColor => true,
         NeverColor => false,
     }