testing: expose Chatty flag #3350

Please provide any additional information below.

After Go 1.

https://groups.google.com/forum/?fromgroups#!topic/golang-dev/-g7k7Hes4ZA

The idea is to allow tests that run like this:

go test -c foo && sudo ./foo.test

or in any other way where the go tool isn't running it, and which has third-party
logging code, to give an indication that the test could disable that logging.