Fix failures in cargo-test

cargo-test just enumerates the test target directory for files
and tries to execute them, but this can pick up the fingerprint
file and fail since it does not have the execute bit set (and
isn't an executable, anyway).  Work around this by skipping dot files.
This could be `.map_or(false, |f| *f.get(0) == b'.')`. ([docs](http://doc.rust-lang.org/master/core/option/type.Option.html#method.map_or))