add failing test for interaction between Read and liquidAssertB

defining a Read instance appears to cause invocations of `liquidAssert` and `liquidAssertB` to refine to `false`.

other typeclasses do not appear to trigger this issue, nor is `liquidError` affected. unsafe type annotations are still detected, e.g.

    {-@ bad :: {v:Integer | v = 5} @-}
    bad = 4