cmd/objdump: mark tests as parallel

Speeds up

go test -short -count=1 cmd/objdump

on my machine from 1.7s to 1.3s.

Not much, but as the backpacking saying goes,
take care of the ounces and the pounds will take care of themselves.

Updates #26473

Change-Id: I59fe9a179e48537c7d82cbba72cde9f92b42a029
Reviewed-on: https://go-review.googlesource.com/c/go/+/176901
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/objdump/objdump_test.go

@@ -87,6 +87,7 @@ var target = flag.String("target", "", "test disassembly of `goos/goarch` binary
 // can handle that one.
 
 func testDisasm(t *testing.T, printCode bool, flags ...string) {
+	t.Parallel()
 	goarch := runtime.GOARCH
 	if *target != "" {
 		f := strings.Split(*target, "/")