Parse tests as private functions.

They actually are private; we can't export / import tests. Fixes #284.

crates/aiken-lang/src/parser.rs

@@ -300,7 +300,7 @@ pub fn test_parser() -> impl Parser<Token, ast::UntypedDefinition, Error = Parse
                 location: span_end,
                 end_position: span.end - 1,
                 name,
-                public: true,
+                public: false,
                 return_annotation: None,
                 return_type: (),
             })