[fail] reset IsThisRendererActing correctly (#16042)

* [fail] reset IsThisRendererActing correctly

I missed this in facebook/react#16039. I'd pointed at the wrong previous state, corrupting it in further use. This PR fixes that, and adds a test to make sure it doesn't happen again.

* warn for unacted effects only in strict mode

src/ReactTestRendererAct.js

@@ -72,7 +72,7 @@ function act(callback: () => Thenable) {
   actingUpdatesScopeDepth++;
   if (__DEV__) {
     previousIsSomeRendererActing = IsSomeRendererActing.current;
-    previousIsThisRendererActing = IsSomeRendererActing.current;
+    previousIsThisRendererActing = IsThisRendererActing.current;
     IsSomeRendererActing.current = true;
     IsThisRendererActing.current = true;
   }