warn if you try to use act() in prod (#16282)

We have behaviour divergence for act() between prod and dev (specifically, act() + concurrent mode does not flush fallbacks in prod. This doesn't affect anyone in OSS yet)

We also don't have a good story for writing tests in prod (and what from what I gather, nobody really writes tests in prod mode).

We could have wiped out act() in prod builds, except that _we_ ourselves use act() for our tests when we run them in prod mode.

This PR is a compromise to all of this. We will log a warning if you try to use act() in prod mode, and we silence it in our test suites.

scripts/jest/shouldIgnoreConsoleError.js

@@ -25,6 +25,16 @@ module.exports = function shouldIgnoreConsoleError(format, args) {
       // They are noisy too so we'll try to ignore them.
       return true;
     }
+    if (
+      format.indexOf(
+        'act(...) is not supported in production builds of React'
+      ) === 0
+    ) {
+      // We don't yet support act() for prod builds, and warn for it.
+      // But we'd like to use act() ourselves for prod builds.
+      // Let's ignore the warning and #yolo.
+      return true;
+    }
   }
   // Looks legit
   return false;