Fix broken tests when supplying test flags

Passing test flags to `go test` (e.g. `go test -test.v`) was breaking
the default handler assertions in TestGo and TestCommander_execute.

Updated `execute` to distinguish between incomingArgs being unset (real
usage) vs. when it is set but empty (default handler test usage).
Merge pull request #6 from mjq/verbose_test

Fix broken tests when supplying test flags