This only breaks in go tip.

This is pretty frustrating: it doesn't look like it's going to be easy
to get the "cover" tool to work across both 1.3 and 1.4, since they
aren't able to agree on where to install the tool from. I'm sure there's
a way to work around this, but in the meantime let's just disable
coverage testing entirely.

The official "cover" package has moved, and Go 1.4 will start enforcing
canonical imports.