cmd/gofmt: make test files self-describing

1) Interpret a comment of the form

//gofmt <flags>

in test files to drive the respective
gofmt command. Eliminates the need to
enumerate all test files in the test
harness.

2) Added -update flag to make it easier
to update test cases.

LGTM=josharian
R=golang-codereviews, josharian
CC=golang-codereviews
https://codereview.appspot.com/130440043