A helpful article on here recommended to call Ruby gems explicitly even though they are built-in.
So instead of
require 'minitest/autorun'
require_relative 'falcon'#file to test
I wrote
require 'rubygems'
gem 'minitest'
require 'minitest/autorun'
require_relative 'falcon'# file to test
The second syntax solved the problems I had.
I am now wondering if the second header is correct, i.e. whether there are any unnecessary repetitions in it.