golang test temp directory

Just to compare vs. what you have with ioutil.TempDir, here's what things look like with io.Reader:

// Load main.conf from the specified file path
func LoadMainSettings(src io.Reader) (*MainSettings, error) {
    b, err := ioutil.ReadAll(src)
    if err != nil { return nil, err }

    r := &MainSettings{}
    err = json.Unmarshal(b, r)
    if err != nil { return nil, err }

    return r, nil
}

Specifically, we change the argument from a path string to a src io.Reader instance, and we replace the ioutil.ReadFile with an ioutil.ReadAll.

The test case that you've written then ends up being a bit shorter precisely because we can dispense with file operations:

s, err := LoadMainSettings(strings.NewReader("{...sample config data...}"))

You could use ioutil.TempDir or TempFile from the same package.

Since Go version 1.15 there is now T.TempDir() in the standard testing package. The docs explain it as follows:

TempDir returns a temporary directory for the test to use. The directory is automatically removed by Cleanup when the test and all its subtests complete. Each subsequent call to t.TempDir returns a unique directory; if the directory creation fails, TempDir terminates the test by calling Fatal.