Set the default log package output to iotuil.Discard during tests if the
`-v` flag isn't set. If we are verbose, then apply the filter according
to the TF_LOG env variable.
This should be quite helpful in debugging aws-sdk-go operations.
Required some tweaking around the `helper/logging` functions to expose an
`IsDebugOrHigher()` helper for us to use.
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.
