Sync rollbackToPreviousProvider tests w/ core (#19428)

This makes it easier to visually compare differences in the
NetworkController unit tests between core and this repo.

test/jest/setup.js

@@ -40,6 +40,45 @@ beforeEach(() => {
 });
 
 expect.extend({
+  /**
+   * Tests that the given promise is fulfilled within a certain amount of time
+   * (which is the default time that Jest tests wait before timing out as
+   * configured in the Jest configuration file).
+   *
+   * Inspired by <https://stackoverflow.com/a/68409467/260771>.
+   *
+   * @param {Promise<any>} promise - The promise to test.
+   * @returns The result of the matcher.
+   */
+  async toBeFulfilled(promise) {
+    if (this.isNot) {
+      throw new Error(
+        "Using `.not.toBeFulfilled(...)` is not supported. Use `.rejects` to test the promise's rejection value instead.",
+      );
+    }
+
+    let rejectionValue = UNRESOLVED;
+    try {
+      await promise;
+    } catch (e) {
+      rejectionValue = e;
+    }
+
+    if (rejectionValue !== UNRESOLVED) {
+      return {
+        message: () =>
+          `Expected promise to be fulfilled, but it was rejected with ${rejectionValue}.`,
+        pass: false,
+      };
+    }
+
+    return {
+      message: () =>
+        'This message should not be displayed as it is for the negative case, which will never happen.',
+      pass: true,
+    };
+  },
+
   /**
    * Tests that the given promise is never fulfilled or rejected past a certain
    * amount of time (which is the default time that Jest tests wait before

types/global.d.ts

@@ -7,6 +7,14 @@ declare class Platform {
 
   closeCurrentWindow: () => void;
 }
+
 export declare global {
   var platform: Platform;
+
+  namespace jest {
+    interface Matchers<R> {
+      toBeFulfilled(): Promise<R>;
+      toNeverResolve(): Promise<R>;
+    }
+  }
 }