test: re-enable rotate tests

Memory usage has been reduced.
The tests are still slow,
but that is issue #10571.

/usr/bin/time shows significant variation
in the peak memory usage compiling with tip.
This is unsurprising, given GC.

Using Go 1.4.2, memory is stable at 410mb.
Using tip at d2ee09298,
memory ranges from 470mb (+15%) to 534mb (+30%),
with a mean of 504mb (+23%), with n=50.

Fixes #9933.

Change-Id: Id31f3ae086ec324abf70e8f1a8044c4a0c27e274
Reviewed-on: https://go-review.googlesource.com/10211Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/rotate0.go

-// skip
-
 // runoutput ./rotate.go
 
 // Copyright 2013 The Go Authors.  All rights reserved.

test/rotate1.go

-// skip
-
 // runoutput ./rotate.go
 
 // Copyright 2013 The Go Authors.  All rights reserved.

test/rotate2.go

-// skip
-
 // runoutput ./rotate.go
 
 // Copyright 2013 The Go Authors.  All rights reserved.

test/rotate3.go

-// skip
-
 // runoutput ./rotate.go
 
 // Copyright 2013 The Go Authors.  All rights reserved.