Tests: MonadCombine->Alternative, add missing ones (#2037)

tests/src/test/scala/cats/tests/AlternativeSuite.scala

@@ -0,0 +1,27 @@
+package cats
+package tests
+
+class AlternativeSuite extends CatsSuite {
+  test("unite") {
+    forAll { (list: List[Option[String]]) =>
+      val expected = list.collect { case Some(s) => s }
+
+      Alternative[List].unite(list) should === (expected)
+    }
+  }
+
+  test("separate") {
+    forAll { (list: List[Either[Int, String]]) =>
+      val ints = list.collect { case Left(i) => i }
+      val strings = list.collect { case Right(s) => s }
+      val expected = (ints, strings)
+
+      Alternative[List].separate(list) should === (expected)
+    }
+  }
+
+  test("guard") {
+    assert(Alternative[Option].guard(true).isDefined)
+    assert(Alternative[Option].guard(false).isEmpty)
+  }
+}