Compilers are one of the easiest and most fun pieces of software to test, because you can test very specific behavior without touching the internals at all.

E.g. if I want to test that `*` has higher precedence than `+`, I would write something like this:

    assert_ast_equals(parse_expr("(a*b)+c"), parse_expr("a*b+c"))
    assert_ast_equals(parse_expr("a+(b*c)"), parse_expr("a+b*c"))

You can rewrite the whole compiler if you want, but as long as you have some notion of a "parser", an "AST" and "two AST nodes being the same" this test will keep working.

This is much more powerful than going into the parser internals and comparing get_operator_precedence('+') with get_operator_precedence('*') which is the default thing you would do if you're told to test every function after writing it.