Protect against spurious failure in ctrl_c test

Protect against spurious failure in ctrl_c test

A failure was witnessed in the Rust repository [1] which happened right after
this test and was a problem removing a directory. Local testing confirms that if
you kill Cargo then right afterwards it's very unlikely to be able to remove the
build directory, presumably because the child process is still getting torn down
in the background.

This commit fixes the ctrl_c test itself to wait for itself to release the bulid
directory, at which point the test has definitely passed.

[1]: https://ci.appveyor.com/project/rust-lang/rust/build/1.0.1331/job/xq4ogmglj7sllibw