Fix: Infinite act loop caused by wrong shouldYield

Fixes the bug demonstrated by the regression test in the previous
commit. Refer to previous message for details.

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -2268,7 +2268,17 @@ function renderRootConcurrent(root: FiberRoot, lanes: Lanes) {
           }
         }
       }
-      workLoopConcurrent();
+
+      if (__DEV__ && ReactCurrentActQueue.current !== null) {
+        // `act` special case: If we're inside an `act` scope, don't consult
+        // `shouldYield`. Always keep working until the render is complete.
+        // This is not just an optimization: in a unit test environment, we
+        // can't trust the result of `shouldYield`, because the host I/O is
+        // likely mocked.
+        workLoopSync();
+      } else {
+        workLoopConcurrent();
+      }
       break;
     } catch (thrownValue) {
       handleThrow(root, thrownValue);