Add a way to provide a prefix to TempFile

This was motivated by the tests tripping up when running in parallel (the default for cargo test). The tests failed intermittently in the following scenario:

• two_files test would create a tempfile then remove a temporary file;
• read test would create a tempfile and able to pick the same name;
• two_files would check if the file was not there anymore, but it was!

Isolating the two_files test with its own prefix fixes the issue.