This will test if deflate output is deterministic between two runs
of the deflater, when write sizes differ.

The deflater makes no official promises that results are
deterministic between runs, but this is a good test to determine
unintentional randomness.

Note that this does not guarantee that results are deterministic
across platforms nor that results will be deterministic between
Go versions. This is also not guarantees we should imply.

Change-Id: Id7dd89fe276060fd83a43d0b34ac35d50fcd32d9
Reviewed-on: https://go-review.googlesource.com/20573Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>