Parse test: Add helper method for asserting parsing failure.
Parse test: Use _assert_parse_fails instead of testing for None.

Once recovery from syntax errors is in place, the parser may no longer
return None when fed a badly-formed program. However, the parser's logger will
always signal the parsing failure correctly.
Parse test: Add test for array non-associativity.
Parse test: Add tests for comparison non-associativity.
Parse test: Add test for assign non-associativity.
Parse test: Move test for precedence of array over function type.
Parse test: Move test for precedence of ref over function type.
Parse test: Rename test for precedence of ref over array type.
Parse test: Move test for associativity of ref type.
Parse test: Move test for associativity of function type.
Parse test: Explain absence of associativity test for array type.