Revert "Work around Dotty #8033: don't test serialization for mixed-in instance"

This reverts commit 327f5e2.

Author: travisbrown

tests/src/test/scala-2.13+/cats/tests/LazyListSuite.scala

@@ -33,8 +33,7 @@ class LazyListSuite extends CatsSuite {
   checkAll("Traverse[LazyList]", SerializableTests.serializable(Traverse[LazyList]))
 
   checkAll("LazyList[Int]", TraverseFilterTests[LazyList].traverseFilter[Int, Int, Int])
-  checkAll("TraverseFilter[LazyList]",
-           SerializableTests.serializable(cats.instances.lazyList.catsStdTraverseFilterForLazyList))
+  checkAll("TraverseFilter[LazyList]", SerializableTests.serializable(TraverseFilter[LazyList]))
 
   checkAll("LazyList[Int]", AlignTests[LazyList].align[Int, Int, Int, Int])
   checkAll("Align[LazyList]", SerializableTests.serializable(Align[LazyList]))

tests/src/test/scala/cats/tests/FunctionSuite.scala

@@ -80,7 +80,7 @@ class FunctionSuite extends CatsSuite {
   checkAll("Contravariant[* => Int]", SerializableTests.serializable(Contravariant[* => Int]))
 
   checkAll("Function1", MonoidKTests[λ[α => α => α]].monoidK[MiniInt])
-  checkAll("MonoidK[λ[α => α => α]", SerializableTests.serializable(cats.instances.function.catsStdMonoidKForFunction1))
+  checkAll("MonoidK[λ[α => α => α]", SerializableTests.serializable(catsStdMonoidKForFunction1))
 
   checkAll("Function1[MiniInt, *]",
            DistributiveTests[MiniInt => *].distributive[Int, Int, Int, Id, Function1[MiniInt, *]])