encoding/xml: parallelize benchmarks

Results remain comparable with the non-parallel version with -cpu=1:
benchmark                old ns/op     new ns/op     delta
BenchmarkMarshal         31220         28618         -8.33%
BenchmarkMarshal-6       37181         7658          -79.40%
BenchmarkUnmarshal       81837         83522         +2.06%
BenchmarkUnmarshal-6     96339         18244         -81.06%

benchmark                old allocs     new allocs     delta
BenchmarkMarshal         23             23             +0.00%
BenchmarkMarshal-6       23             23             +0.00%
BenchmarkUnmarshal       189            189            +0.00%
BenchmarkUnmarshal-6     189            189            +0.00%

benchmark                old bytes     new bytes     delta
BenchmarkMarshal         5776          5776          +0.00%
BenchmarkMarshal-6       5776          5776          +0.00%
BenchmarkUnmarshal       8576          8576          +0.00%
BenchmarkUnmarshal-6     8576          8576          +0.00%

updates #18177

Change-Id: I7e7055a11d18896bd54d7d773f2ec64767cdb4c8
Reviewed-on: https://go-review.googlesource.com/36810
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/marshal_test.go

@@ -1901,17 +1901,21 @@ func TestMarshalFlush(t *testing.T) {
 
 func BenchmarkMarshal(b *testing.B) {
 	b.ReportAllocs()
-	for i := 0; i < b.N; i++ {
-		Marshal(atomValue)
-	}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			Marshal(atomValue)
+		}
+	})
 }
 
 func BenchmarkUnmarshal(b *testing.B) {
 	b.ReportAllocs()
 	xml := []byte(atomXml)
-	for i := 0; i < b.N; i++ {
-		Unmarshal(xml, &Feed{})
-	}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			Unmarshal(xml, &Feed{})
+		}
+	})
 }
 
 // golang.org/issue/6556