doc.go contains the details. The short story:
- command line is passed to the binary
- a new flag, -file, is needed to name files
- known flags have the "test." prefix added for convenience.
- gotest-specific flags are trimmed from the command line.

The effect should be that most existing uses are unaffected,
the ability to name files is still present, and it's nicer to use.
The downside is a lot more code in gotest.

Also allow a test to be called just Test.

R=rsc, niemeyer, rog, r2
CC=golang-dev
https://golang.org/cl/4307049