encoding/json: make BenchmarkSkipValue more stable
encoding/json: make BenchmarkSkipValue more stable

BenchmarkSkipValue was sensitive to the value of
b.N due to its significant startup cost.

Two adjacent runs before this CL:

BenchmarkSkipValue	      50	  21047499 ns/op	  93.37 MB/s
BenchmarkSkipValue	     100	  17260554 ns/op	 118.05 MB/s

After this CL, using benchtime to recreate the
difference in b.N:

BenchmarkSkipValue	      50	  15204797 ns/op	 131.67 MB/s
BenchmarkSkipValue	     100	  15332319 ns/op	 130.58 MB/s

Change-Id: Iac86f86dd774d535302fa5e4c08f89f8da00be9e
Reviewed-on: https://go-review.googlesource.com/10053
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

jimsmart/ejson