os: use t.TempDir in TestReadOnlyWriteFile

This test is checking WriteFile, not MkdirTemp, and using t.TempDir
makes the test case code a tad smaller and simpler.

Change-Id: I48837f77572f375d56f3a387efa9a4ee1d3706fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/606898


Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/os/read_test.go

@@ -78,16 +78,11 @@ func TestReadOnlyWriteFile(t *testing.T) {
 	t.Parallel()
 
 	// We don't want to use CreateTemp directly, since that opens a file for us as 0600.
-	tempDir, err := MkdirTemp("", t.Name())
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer RemoveAll(tempDir)
-	filename := filepath.Join(tempDir, "blurp.txt")
+	filename := filepath.Join(t.TempDir(), "blurp.txt")
 
 	shmorp := []byte("shmorp")
 	florp := []byte("florp")
-	err = WriteFile(filename, shmorp, 0444)
+	err := WriteFile(filename, shmorp, 0444)
 	if err != nil {
 		t.Fatalf("WriteFile %s: %v", filename, err)
 	}