Fix servant-client tests to properly use hspec-discover.

For some reason hspec-discover wasn't enabled for servant-client,
causing test cases to be skipped and fasely reporting servant-client as
test-passing whilst it was actually supposed to fail the tests.

For example, we redefined BaseUrl recently to have a path field, but the
tests don't reflect that yet but passed anyway. Because the test case
wasn't even discovered and run.

servant-client/test/Spec.hs

@@ -1,7 +1,2 @@
-import           Servant.ClientSpec (failSpec, spec)
-
-main :: IO ()
-main = do
-  spec
-  failSpec
+{-# OPTIONS_GHC -F -pgmF hspec-discover #-}