all: use t.TempDir in tests

This removes all usage of ioutil.TempDir in tests (and a few cases of
os.TempDir as well, while we're at it), which simplifies test cleanup a
lot.

Compile tested (go test -c) for most platforms (except zos).

Change-Id: I9178f5ec615c0a6cfef36e8de78c6b72e055b7cb
Reviewed-on: https://go-review.googlesource.com/c/sys/+/526297
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: qiulaidongfeng <2645477756@qq.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux_test.go

@@ -986,13 +986,7 @@ func TestOpenat2(t *testing.T) {
 	}
 
 	// prepare
-	tempDir, err := ioutil.TempDir("", t.Name())
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(tempDir)
-
-	subdir := filepath.Join(tempDir, "dir")
+	subdir := filepath.Join(t.TempDir(), "dir")
 	if err := os.Mkdir(subdir, 0755); err != nil {
 		t.Fatal(err)
 	}