Jest: Add browser.runtime (webextension-polyfill) utils tests (#16483)

* jest: add browser-runtime.utils tests

* browser-runtime.utils: rm checkForLastErrorAndWarn
- will be removing use in #16488

shared/modules/browser-runtime.utils.test.js

@@ -0,0 +1,54 @@
+import sinon from 'sinon';
+import browser from 'webextension-polyfill';
+import log from 'loglevel';
+import * as BrowserRuntimeUtil from './browser-runtime.utils';
+
+const mockLastError = { message: 'error', stack: [] };
+
+describe('Browser Runtime Utils', () => {
+  beforeAll(() => {
+    sinon.replace(browser, 'runtime', {
+      lastError: undefined,
+    });
+  });
+
+  describe('checkForLastError', () => {
+    it('should return undefined if no lastError found', () => {
+      expect(BrowserRuntimeUtil.checkForLastError()).toBeUndefined();
+    });
+
+    it('should return the lastError (Error object) if lastError is found', () => {
+      sinon.stub(browser.runtime, 'lastError').value(mockLastError);
+
+      expect(BrowserRuntimeUtil.checkForLastError()).toStrictEqual(
+        mockLastError,
+      );
+    });
+
+    it('should return an Error object if the lastError is found with no stack', () => {
+      sinon
+        .stub(browser.runtime, 'lastError')
+        .value({ message: mockLastError.message });
+
+      const result = BrowserRuntimeUtil.checkForLastError();
+
+      expect(result).toStrictEqual(expect.any(Error));
+      expect(result).toHaveProperty('stack');
+      expect(result.message).toBe(mockLastError.message);
+    });
+  });
+
+  describe('checkForLastErrorAndLog', () => {
+    it('should log and return error if error was found', () => {
+      sinon.stub(browser.runtime, 'lastError').value({ ...mockLastError });
+      sinon.stub(log, 'error');
+
+      const result = BrowserRuntimeUtil.checkForLastErrorAndLog();
+
+      expect(log.error.calledWith(result)).toBeTruthy();
+      expect(result).toStrictEqual(mockLastError);
+
+      log.error.restore();
+    });
+  });
+});