testing: fix output formatting

Revision 5e7fd762f356 has changed the output formatting in a way that
is no longer in line with the format described by the revision
ff0ade0b937b which has introduced this functionality.
When decorating the first line, instead of indenting the whole line,
the current implementation adds indentation right after the "decorate"
part and  before the "log" message.
The fix addresses this issue.

R=golang-dev, iant, minux.ma, r, rsc, remyoudompheng
CC=golang-dev
https://codereview.appspot.com/7304094

Committer: Rob Pike <r@golang.org>