regexp: fix BenchmarkMatch_onepass_regex

This benchmark - in contrast to all other benchmarks - was
running the regexp match on 1-byte substrings of the input
instead of the entire input. Worse, it was doing so by preallocating
a slice of slices of every 1-byte substring. Needless to say,
this does not accurately reflect what happens when the regexp
matcher is given a large input.

Change-Id: Icd5b95f0e43f554a6b93164916745941366e03d6
Reviewed-on: https://go-review.googlesource.com/c/139778
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/regexp/exec_test.go

@@ -692,18 +692,12 @@ func BenchmarkMatch_onepass_regex(b *testing.B) {
 			continue
 		}
 		t := makeText(size.n)
-		bs := make([][]byte, len(t))
-		for i, s := range t {
-			bs[i] = []byte{s}
-		}
 		b.Run(size.name, func(b *testing.B) {
 			b.SetBytes(int64(size.n))
 			b.ReportAllocs()
 			for i := 0; i < b.N; i++ {
-				for _, byts := range bs {
-					if !r.Match(byts) {
-						b.Fatal("not match!")
-					}
+				if !r.Match(t) {
+					b.Fatal("not match!")
 				}
 			}
 		})