Document the optimistic runtime parser behavior in tests

[stasm]

There's a test in fluent/test/primitives_test.js which uses invalid syntax:

selector-attr = { baz.attr ->
    [FooBarBazAttribute] FooBarBaz
   *[other] Other
}

The spec forbids AttributeExpressions as selectors. This test passes, however, because it's a FluentBundle test, and FluentBundle uses the optimistic runtime parser (#289). The optimistic parser focuses on well-formed-ness and doesn't reject this syntax as invalid.

We should fix this test to use a term attribute (-baz.attr). We should also review other tests and we should document other parsing differences like this one by writing more tests.