[dev.fuzz] internal/fuzz: add more benchmarks for workers

* Benchmark{Marshal,Unmarshal}CorpusFile - measures time it takes to
  serialize and deserialize byte slices of various lengths.
* BenchmarkWorkerPing - spins up a worker and measures time it takes
  to ping it N times as a rough measure of RPC latency.
* BenchmarkWorkerFuzz - spins up a worker and measures time it takes
  to mutate an input and call a trivial fuzz function N times.

Also a few small fixes to make this easier.

Change-Id: Id7f2dc6c6c05005cf286f30e6cc92a54bf44fbf7
Reviewed-on: https://go-review.googlesource.com/c/go/+/333670
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

src/internal/fuzz/encoding_test.go

@@ -5,6 +5,7 @@
 package fuzz
 
 import (
+	"strconv"
 	"strings"
 	"testing"
 )
@@ -120,3 +121,44 @@ float32(2.5)`,
 		})
 	}
 }
+
+// BenchmarkMarshalCorpusFile measures the time it takes to serialize byte
+// slices of various sizes to a corpus file. The slice contains a repeating
+// sequence of bytes 0-255 to mix escaped and non-escaped characters.
+func BenchmarkMarshalCorpusFile(b *testing.B) {
+	buf := make([]byte, 1024*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i)
+	}
+
+	for sz := 1; sz <= len(buf); sz <<= 1 {
+		sz := sz
+		b.Run(strconv.Itoa(sz), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				b.SetBytes(int64(sz))
+				marshalCorpusFile(buf[:sz])
+			}
+		})
+	}
+}
+
+// BenchmarkUnmarshalCorpusfile measures the time it takes to deserialize
+// files encoding byte slices of various sizes. The slice contains a repeating
+// sequence of bytes 0-255 to mix escaped and non-escaped characters.
+func BenchmarkUnmarshalCorpusFile(b *testing.B) {
+	buf := make([]byte, 1024*1024)
+	for i := 0; i < len(buf); i++ {
+		buf[i] = byte(i)
+	}
+
+	for sz := 1; sz <= len(buf); sz <<= 1 {
+		sz := sz
+		data := marshalCorpusFile(buf[:sz])
+		b.Run(strconv.Itoa(sz), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				b.SetBytes(int64(sz))
+				unmarshalCorpusFile(data)
+			}
+		})
+	}
+}