test: use T.TempDir to create temporary test directory (#21043)

A testing cleanup. 

This pull request replaces `os.MkdirTemp` with `t.TempDir`. We can use the `T.TempDir` function from the `testing` package to create temporary directory. The directory created by `T.TempDir` is automatically removed when the test and all its subtests complete. 

This saves us at least 2 lines (error check, and cleanup) on every instance, or in some cases adds cleanup that we forgot.

Reference: https://pkg.go.dev/testing#T.TempDir

```go
func TestFoo(t *testing.T) {
	// before
	tmpDir, err := os.MkdirTemp("", "")
	require.NoError(t, err)
	defer os.RemoveAll(tmpDir)

	// now
	tmpDir := t.TempDir()
}
```

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

tests/integration/git_clone_wiki_test.go

@@ -35,8 +35,7 @@ func TestRepoCloneWiki(t *testing.T) {
 	onGiteaRun(t, func(t *testing.T, u *url.URL) {
 		defer tests.PrepareTestEnv(t)()
 
-		dstPath, err := os.MkdirTemp("", "clone_wiki")
-		assert.NoError(t, err)
+		dstPath := t.TempDir()
 
 		r := fmt.Sprintf("%suser2/repo1.wiki.git", u.String())
 		u, _ = url.Parse(r)