compress/flate: simplify using subtests and sub-benchmarks

compress/flate: simplify using subtests and sub-benchmarks

This causes the large files to be loaded only once per benchmark.

This CL also serves as an example use case of sub(tests|-benchmarks).

This CL ensures that names are identical to the original
except for an added slashes. Things could be
simplified further if this restriction were dropped.

Change-Id: I45e303e158e3152e33d0d751adfef784713bf997
Reviewed-on: https://go-review.googlesource.com/23420
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>