Add missing unit tests

[luc-tielen]

At the moment the test suite is fairly complete for the current feature set, but some things are still missing:

• Test that checks a program with both a recursive rule and a non recursive rule that uses the recursive rule in it's body.
• Test for a rule with more than 2 rule clauses.
• Parsing tests, most notably tests with invalid programs. These should check that a clear error message is returned.

These should all be tested in a similar way to tests in the tests/lowering/ directory, in a separate test file in that directory.

• Add a test that checks the caching mechanism works and e.g. btree code is reused. A good candidate for this is the single_non_recursive_rule test.
• A test for the symbol table?