Better testing for the faker combinators

[psibi]

We currently use verifyFakes to test the combinators like this:

verifyFakes :: [Fake Text] -> IO [Bool]
verifyFakes funs = do
  let fs :: [IO Text] = map (generateWithSettings fakerSettings) funs
      gs :: [IO Bool] = map (\f -> isText <$> f) fs
  sequence gs

Instead, we should be using quickcheck to generate the dataset. We already have quickcheck as the test dependency, so it shouldn't be an issue. Also, checking for non null property and different samples should be good enough. Example for non null property: https://hackage.haskell.org/package/fakedata-quickcheck-0.2.0/docs/Test-QuickCheck-Gen-Faker.html

Example for different samples: https://github.com/fakedata-haskell/fakedata-quickcheck/blob/e6d10ad99796665d8041dbbbd3764407b14116f5/test/Test/FakeQuickCheckSpec.hs#L32