test: avoid interface conversion in rotate.go
test: avoid interface conversion in rotate.go

It is not necessary for the test to be effective and uses a
lot of resources in the compiler. Memory usage is halved and
compilation around 8x faster.

R=golang-dev, r, rsc, r
CC=golang-dev
http://codereview.appspot.com/6290044

pombredanne/go-deleteme,dsriram/golang,apparentlymart/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,tv42/old-go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go