We weren't doing any log setup for acceptance tests, which made it
difficult to wrangle log output in CI.

This moves the log setup functions we use in `main` over into a helper
package so we can use them for acceptance tests as well.

This means that acceptance tests will by default be a _lot_ quieter,
only printing out actual test output. Setting `TF_LOG=trace` will
restore the full prior noise level.

Only minor behavior change is to make `ioutil.Discard` the default
return value rather than a `nil` that needs to be checked for.