net/http: deflake TestResponseWriterWriteStringAllocs, test interface instead

Skip the allocation testing (which was only used as a signal for
whether the interface was implemented by ResponseWriter), and just
test for it directly.

Fixes #9575

Change-Id: Ie230f1d21b104537d5647e9c900a81509d692469
Reviewed-on: https://go-review.googlesource.com/2720


Reviewed-by: Alex Brainman <alex.brainman@gmail.com>