archive/zip: parallelize benchmarks
Add subbenchmarks for BenchmarkZip64Test with different sizes to tease
apart construction costs vs. steady-state throughput.
Results remain comparable with the non-parallel version with -cpu=1:
benchmark old ns/op new ns/op delta
BenchmarkCompressedZipGarbage
26832835 27506953 +2.51%
BenchmarkCompressedZipGarbage-6
27172377 4321534 -84.10%
BenchmarkZip64Test
196758732 197765510 +0.51%
BenchmarkZip64Test-6
193850605 192625458 -0.63%
benchmark old allocs new allocs delta
BenchmarkCompressedZipGarbage 44 44 +0.00%
BenchmarkCompressedZipGarbage-6 44 44 +0.00%
benchmark old bytes new bytes delta
BenchmarkCompressedZipGarbage 5592 5664 +1.29%
BenchmarkCompressedZipGarbage-6 5592 21946 +292.45%
updates #18177
Change-Id: Icfa359d9b1a8df5e085dacc07d2b9221b284764c
Reviewed-on: https://go-review.googlesource.com/36719
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>