Float is currently being tested only with the external runtime. This runtime is unstable and should not be part of stable builds and we should still be testing the integrated Fizz runtime. This change forks the Float tests to ensure we test with and without the external Fizz runtime

Author: gnoff

packages/react-dom/src/__tests__/ReactDOMFizzServer-test.js

@@ -153,7 +153,7 @@ describe('ReactDOMFizzServer', () => {
     });
 
     renderOptions = {};
-    if (gate(flags => flags.enableFizzExternalRuntime)) {
+    if (gate(flags => flags.shouldUseFizzExternalRuntime)) {
       renderOptions.unstable_externalRuntimeSrc =
         'react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js';
     }
@@ -610,7 +610,7 @@ describe('ReactDOMFizzServer', () => {
         Array.from(container.getElementsByTagName('script')).filter(
           node => node.getAttribute('nonce') === CSPnonce,
         ).length,
-      ).toEqual(6);
+      ).toEqual(gate(flags => flags.shouldUseFizzExternalRuntime) ? 6 : 5);
 
       await act(() => {
         resolve({default: Text});
@@ -4292,7 +4292,7 @@ describe('ReactDOMFizzServer', () => {
     );
   });
 
-  // @gate enableFizzExternalRuntime
+  // @gate shouldUseFizzExternalRuntime
   it('does not send script tags for SSR instructions when using the external runtime', async () => {
     function App() {
       return (

packages/react-dom/src/__tests__/ReactDOMFloat-test.js

@@ -97,7 +97,7 @@ describe('ReactDOMFloat', () => {
     });
 
     renderOptions = {};
-    if (gate(flags => flags.enableFizzExternalRuntime)) {
+    if (gate(flags => flags.shouldUseFizzExternalRuntime)) {
       renderOptions.unstable_externalRuntimeSrc =
         'react-dom/unstable_server-external-runtime';
     }

scripts/jest/TestFlags.js

@@ -83,6 +83,15 @@ function getTestFlags() {
       enableSuspenseList: releaseChannel === 'experimental' || www,
       enableLegacyHidden: www,
 
+      // This flag is used to determine whether we should run Fizz tests using
+      // the external runtime or the inline script runtime.
+      // For Meta we use variant to gate the feature. For OSS we use experimental
+      shouldUseFizzExternalRuntime: !featureFlags.enableFizzExternalRuntime
+        ? false
+        : www
+        ? __VARIANT__
+        : __EXPERIMENTAL__,
+
       // This is used by useSyncExternalStoresShared-test.js to decide whether
       // to test the shim or the native implementation of useSES.
       // TODO: It's disabled when enableRefAsProp is on because the JSX