We need not only “best practices” and “clever” categories, but also something like “I want to hurt myself.”

The current tests aren't very informative. This would be a slight improvement:

input = ["This is an alpha line", "Beta line next!", "Another AlphA", "I have no idea", nil]
Test.assert_equals(line_types(input), [:alpha, :beta, :alpha, :unknown, :unknown])

As it stands, you only know if its pass/fail. Using the above would show you which of the elements in the returned array are wrong.