Add abstractions for backend tests

One function that can be used to test conversion functions, and one that
can be used to test compilation functions.
Add compileIntegerLiteral to backend

It’s a bit strange that the grammar rule is 'nonstringLiteral' (includes
character and float literals).
redhat backend: Add tests for IntegerLiteral functions

For both conversion and compilation, using the new helpers introduced in
7836855.