yampa-test: Imports from applicative in old GHCs. Refs #239.

The tests use <$> and <*> but do not import them appropriately in old
GHC versions, which causes compilation to fail.

This commit adds a conditional import of those definitions when using
versions of base that do not include them in the Prelude.

yampa-test/tests/Test/FRP/Yampa/Event.hs

@@ -1,3 +1,4 @@
+{-# LANGUAGE CPP #-}
 -- |
 -- Description : Test cases for events
 -- Copyright   : Ivan Perez, 2022
@@ -10,6 +11,11 @@ module Test.FRP.Yampa.Event
 import Control.Applicative ((<|>))
 import Control.Monad       (guard, join)
 
+#if !MIN_VERSION_base(4,8,0)
+import Control.Applicative (pure, (<*>))
+import Data.Functor        ((<$>))
+#endif
+
 import Test.QuickCheck       hiding (once, sample)
 import Test.Tasty            (TestTree, testGroup)
 import Test.Tasty.QuickCheck (testProperty)