Enable tests from `IDLGrammarSpec.'positive single service with parameters #methodName'`

[tglaeser]

Currently IDLGrammarSpecification.'positive single service with parameters #methodName' is disabled as test are failing since merges for #18.

The string representation of the actual and expected program looks the same:

record {
    preimage: vec nat8;
    image: vec nat8;
};
type List = opt record {
    Key;
    List;
};
type Bucket = List;
type List_2 = opt record {
    text;
    List_2;
};
service : {
    "peers": () -> (List_2);
}

However the test expects field List/List_2 to be of type IDLType.Id while it is in fact of IDLType.Primitive.Null.

The root-cause seems to be related to the IDLGrammar#ids workaround, see discussions on #20 and issue #21.