Add a "msg" parameter to testutil.Equals.

[krasi-georgiev]

In most cases we end up doing something like:

// Blocks should have been created .....
testutil.Equals(t, 0, len(db.blocks))

Instead it would be better if we have something like:

testutil.Equals(t, 0, len(db.blocks), "Blocks didn't get created")

This would help when looking at the output of the failing tests.