haskellPackages.ghc84x.singletons: disable test suite for ghc-8.4.3

Otherwise, the test suite fails, due to the 'Lambdas' test failing:

    Lambdas:              FAIL (1.06s)
      42c42
      <     foo8 x = (\ Foo a _ -> a) x
      ---
      >     foo8 x = (\ (Foo a _) -> a) x

I assume this is a minor bogus failure due to some change in 8.4.3 that
nobody expected.

Signed-off-by: Austin Seipp <aseipp@pobox.com>