math/bits: move tests into their own package

math/bits: move tests into their own package

This makes math/bits not have any explicit imports even
when compiling tests and thereby avoids import cycles when
dependencies of testing want to import math/bits.

Change-Id: I95eccae2f5c4310e9b18124abfa85212dfbd9daa
Reviewed-on: https://go-review.googlesource.com/110479
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>