Auto merge of #2496 - alexcrichton:fix-concurrent, r=alexcrichton

Relax an assertion in the concurrent tests

The output may also have information about blocking, we just want the successful
part.

tests/test_cargo_concurrent.rs

@@ -86,7 +86,7 @@ test!(one_install_should_be_bad {
     assert_that(bad, execs().with_status(101).with_stderr_contains(&format!("\
 {error} binary `foo[..]` already exists in destination as part of `[..]`
 ", error = ERROR)));
-    assert_that(good, execs().with_status(0).with_stderr("\
+    assert_that(good, execs().with_status(0).with_stderr_contains("\
 warning: be sure to add `[..]` to your PATH [..]
 "));