test: set react flags in spec files only

Author: littensy

src/init.spec.ts

@@ -0,0 +1,7 @@
+_G.__ROACT_17_MOCK_SCHEDULER__ = true;
+
+export = () => {
+	afterAll(() => {
+		_G.__ROACT_17_MOCK_SCHEDULER__ = undefined;
+	});
+};

src/react-env.d.ts

@@ -1,4 +1,4 @@
 interface _G {
-	__DEV__: boolean;
-	__ROACT_17_MOCK_SCHEDULER__: boolean;
+	__DEV__?: boolean;
+	__ROACT_17_MOCK_SCHEDULER__?: boolean;
 }

src/utils/testez.tsx

@@ -1,5 +1,3 @@
-_G.__ROACT_17_MOCK_SCHEDULER__ = true;
-
 import React, { useRef } from "@rbxts/react";
 import { act, createLegacyRoot } from "@rbxts/react-roblox";