charm: avoid use of /bin/cp

Isolated tests should avoid use of external commands when possible.

Also update utils dependency so that we can use the new fs package
containing the Copy function.
charm/testing: do not rely on copy overwriting
Merge pull request #57 from rogpeppe/006-charm-no-cp

Isolated tests should avoid use of external commands when possible.

Also update utils dependency so that we can use the new fs package
containing the Copy function.