Test that immovable types are allowed in Maybe

Immovable types can be already wrapped into `Maybe`, which can be constructed as default `nullptr`, set to a constructed value via `.emplace()`, but couldn't be cleared back to `nullptr`. The previous commit fixed the bug, this commit adds a test.

c++/src/kj/common-test.c++

@@ -45,6 +45,11 @@ struct ImplicitToInt {
   }
 };
 
+struct Immovable {
+  Immovable() = default;
+  KJ_DISALLOW_COPY(Immovable);
+};
+
 TEST(Common, Maybe) {
   {
     Maybe<int> m = 123;
@@ -63,6 +68,23 @@ TEST(Common, Maybe) {
     EXPECT_EQ(123, m.orDefault(456));
   }
 
+  {
+    Maybe<int> m = 0;
+    EXPECT_FALSE(m == nullptr);
+    EXPECT_TRUE(m != nullptr);
+    KJ_IF_MAYBE(v, m) {
+      EXPECT_EQ(0, *v);
+    } else {
+      ADD_FAILURE();
+    }
+    KJ_IF_MAYBE(v, mv(m)) {
+      EXPECT_EQ(0, *v);
+    } else {
+      ADD_FAILURE();
+    }
+    EXPECT_EQ(0, m.orDefault(456));
+  }
+
   {
     Maybe<int> m = nullptr;
     EXPECT_TRUE(m == nullptr);
@@ -216,6 +238,16 @@ TEST(Common, Maybe) {
       ADD_FAILURE();
     }
   }
+
+  {
+    // Test usage of immovable types.
+    Maybe<Immovable> m;
+    KJ_EXPECT(m == nullptr);
+    m.emplace();
+    KJ_EXPECT(m != nullptr);
+    m = nullptr;
+    KJ_EXPECT(m == nullptr);
+  }
 }
 
 TEST(Common, MaybeConstness) {