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>
