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/10053Reviewed-by: Andrew Gerrand <adg@golang.org>

src/encoding/json/scanner_test.go

@@ -209,6 +209,7 @@ var benchScan scanner
 
 func BenchmarkSkipValue(b *testing.B) {
 	initBig()
+	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
 		nextValue(jsonBig, &benchScan)
 	}