cmd/internal/obj/x86: avoid os.Chdir in issue19518_test.go

Chdir leaves the test in the wrong working directory if objdumpOutput
calls t.Fatalf (or panics), and it isn't necessary here anyway. Set
the Dir field on the commands instead.

Change-Id: I9f0eb0d4f8d15043f1e13472126ca1a1ce4b7cb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/167081
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/internal/obj/x86/issue19518_test.go

@@ -32,10 +32,6 @@ func main() {
 `
 
 func objdumpOutput(t *testing.T) []byte {
-	cwd, err := os.Getwd()
-	if err != nil {
-		t.Fatal(err)
-	}
 	tmpdir, err := ioutil.TempDir("", "19518")
 	if err != nil {
 		t.Fatal(err)
@@ -63,15 +59,13 @@ func objdumpOutput(t *testing.T) []byte {
 	if err != nil {
 		t.Fatal(err)
 	}
-	err = os.Chdir(tmpdir)
-	if err != nil {
-		t.Fatal(err)
-	}
+
 	cmd := exec.Command(
 		testenv.GoToolPath(t), "build", "-o",
 		filepath.Join(tmpdir, "output"))
 
 	cmd.Env = append(os.Environ(), "GOARCH=amd64", "GOOS=linux")
+	cmd.Dir = tmpdir
 
 	out, err := cmd.CombinedOutput()
 	if err != nil {
@@ -81,14 +75,12 @@ func objdumpOutput(t *testing.T) []byte {
 		testenv.GoToolPath(t), "tool", "objdump", "-s", "testASM",
 		filepath.Join(tmpdir, "output"))
 	cmd2.Env = cmd.Env
+	cmd2.Dir = tmpdir
 	objout, err := cmd2.CombinedOutput()
 	if err != nil {
 		t.Fatalf("error %s output %s", err, objout)
 	}
-	err = os.Chdir(cwd)
-	if err != nil {
-		t.Fatal(err)
-	}
+
 	return objout
 }