Test run-pass-fulldeps\issue-15149.rs usually fails on Windows

[petrochenkov]

https://github.com/rust-lang/rust/blob/master/src/test/run-pass-fulldeps/issue-15149.rs

Here's the problematic piece:

    let child_output = process::Command::new("mytest").env("PATH", &path)
                                                      .arg("child")
                                                      .output().unwrap();

    fs::remove_dir_all(&child_dir).unwrap(); // The panic happens here

On my machine remove_dir_all usually can't delete file mytest.exe with "access denied" error. If I insert something like std::thread::sleep_ms(1000); before remove_dir_all then the test consistently passes. Supposedly we are trying to delete it too soon when it's still locked after execution.