GrammarKit is a plugin by JetBrains for creating custom language plugins
for JetBrains IDEs (such as IntelliJ, RubyMine, CLion and more). It
defines a BNF parser language which can be used to generate a parser in
Java, and it also integrates JFLex for generating a lexer in Java.
Both of these generated Java files can be recognised by a comment on the
first line of the file, and so classifying them as generated is trivial.
This gives us a consistent test framework across all Ruby versions which
should help avoid errors that are only found when CI runs the tests on
different Rubies. (And this fixes an immediate bug where there's no
`skip` method in the version of test-unit we're currently using only on
Ruby 2.2.)
