Ruby 3.0 should be enabled, read this to learn how to do it

Please organize the structure of test fixture as following (although it has been mentioned in the attached link, I'm repeated here again)

describe "<This message should describe the categories of test groups inside this block>" do
  it "<A short message describing this test group>" do
    expect(...).to eq(...) #Assertions
    #Or Test.assert_equals(user_response, reference_response)
  end
end

Given a board of NxN, distributed with tiles labeled 2 to N^2 - 1 (inclusive)

Ehm, no. That would be 0 to n²-1 ( and the second sample tests has two threes and no two ).

Did you mix this up with the minimum value for N ?