The `testutils` module should ideally not be part of the library
dependencies. Since they're used by the integration tests (and the CLI
tests), we need to move them to a separate crate to achieve that.
Unlike the other places I fixed in 134940d2bb, the calls in
`working_copy.rs` should not simply use an existing file if the target
file was open. They should probably try again instead, but I'll leave
that for later.
On Windows, it seems that you can't rename a file if the target file
is open (Stebalien/tempfile#131). I think that's the reason for our
failing tests on Windows. This patch adds a simple wrapper around
`NamedTempFile::persist()` that returns the existing file instead of
failing, if there is one.
