Fix deprecations of global expectations for Minitest 6

[JuanitoFatas]

Howdy,

I was running the test for this project. Saw a lot of warnings from Minitest.

The deprecation was introduced by this commit.

There are 3 syntax to choose from:

_(1 + 1).must_equal 2
value(1 + 1).must_equal 2
expect(1 + 1).must_equal 2

I pick the shortest one, I could update based on your preference.

Thanks.

[henrik]

Thank you so much! :)