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/36719Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>