Mark readLogFile as a test helper function

[farshidtz]

This could be marked as a helper function:

func readLogFile(t *testing.T, filePath string) (string, error) {
    t.Helper()
    text, err := os.ReadFile(filePath)
    if err != nil {
        return "", err
    }
    return string(text), nil
}

(then change the in the call of the function also to: logs, err := readLogFile(t, appLogPath) )

Originally posted by @locnnil in #36 (comment)

[farshidtz]

This function doesn't perform much. The os.ReadFile function can be called directly by waitForLogMessage. On the other hand, waitForLogMessage (the caller) could be marked as helper and moved to canonical/matter-snap-testing.

[farshidtz]

The function has been removed in #45

More refactoring is needed, as suggested in the above comment.