path: add Join benchmark

This is a case where CL 653856 saves an allocation.

        │     old     │                 new                 │
        │   sec/op    │   sec/op     vs base                │
Join-24   73.57n ± 1%   60.27n ± 1%  -18.07% (p=0.000 n=10)

        │    old     │                new                 │
        │    B/op    │    B/op     vs base                │
Join-24   48.00 ± 0%   24.00 ± 0%  -50.00% (p=0.000 n=10)

        │    old     │                new                 │
        │ allocs/op  │ allocs/op   vs base                │
Join-24   2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)

Change-Id: I56308262ca73a7ab9698b54fd8681f5b44626995
Reviewed-on: https://go-review.googlesource.com/c/go/+/665075
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

src/path/path_test.go

@@ -5,6 +5,7 @@
 package path_test
 
 import (
+	"path"
 	. "path"
 	"runtime"
 	"testing"
@@ -234,3 +235,14 @@ func TestIsAbs(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkJoin(b *testing.B) {
+	b.ReportAllocs()
+	parts := []string{"one", "two", "three", "four"}
+	s := parts[0]
+	for b.Loop() {
+		parts[0] = s
+		s = path.Join(parts...)
+		s = s[:3]
+	}
+}