testing,go-test: document the behavior of testing.Log{f} #5174

by ian.nevir.net:

go test eats output on successful runs; which can be confusing if you're not aware of
that (particularly when trying to debug init() code).  This behavior doesn't appear to
be directly documented anywhere.

go help test, and probably the testing package, should document it.  The only
documentation I can find about this is hidden away in go help testflags, and that's only
indirect documentation (the -v flag)