test: use T.TempDir to create temporary test directory

This commit replaces `ioutil.TempDir` with `t.TempDir` in tests. The
directory created by `t.TempDir` is automatically removed when the test
and all its subtests complete.

Prior to this commit, temporary directory created using `ioutil.TempDir`
needs to be removed manually by calling `os.RemoveAll`, which is omitted
in some tests. The error handling boilerplate e.g.
	defer func() {
		if err := os.RemoveAll(dir); err != nil {
			t.Fatal(err)
		}
	}
is also tedious, but `t.TempDir` handles this for us nicely.

Reference: https://pkg.go.dev/testing#T.TempDir
Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

libpod/container_internal_linux_test.go

@@ -4,8 +4,6 @@
 package libpod
 
 import (
-	"io/ioutil"
-	"os"
 	"testing"
 
 	spec "github.com/opencontainers/runtime-spec/specs-go"
@@ -13,12 +11,6 @@ import (
 )
 
 func TestGenerateUserPasswdEntry(t *testing.T) {
-	dir, err := ioutil.TempDir("", "libpod_test_")
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer os.RemoveAll(dir)
-
 	c := Container{
 		config: &ContainerConfig{
 			Spec: &spec.Spec{},