add commHelper.test.ts

-Added one test for `fetchUrlCached` - uses a mocked 'fetch' to simulate retreieving data from a URL. It ensures that the second time this URL is queried, the data comes back faster (because it gets cached in the localStorage after the first call)

Note at the bottom explains why other functions were not tested.

www/__tests__/commHelper.test.ts

@@ -0,0 +1,41 @@
+import { mockLogger } from '../__mocks__/globalMocks';
+import { fetchUrlCached } from '../js/commHelper';
+
+mockLogger();
+
+// mock for JavaScript 'fetch'
+// we emulate a 100ms delay when i) fetching data and ii) parsing it as text
+global.fetch = (url: string) => new Promise((rs, rj) => {
+  setTimeout(() => rs({
+    text: () => new Promise((rs, rj) => {
+      setTimeout(() => rs('mock data for ' + url), 100);
+    })
+  }));
+}) as any;
+
+it('fetches text from a URL and caches it so the next call is faster', async () => {
+  const tsBeforeCalls = Date.now();
+  const text1 = await fetchUrlCached('https://raw.githubusercontent.com/e-mission/e-mission-phone/master/README.md');
+  const tsBetweenCalls = Date.now();
+  const text2 = await fetchUrlCached('https://raw.githubusercontent.com/e-mission/e-mission-phone/master/README.md');
+  const tsAfterCalls = Date.now();
+  expect(text1).toEqual(expect.stringContaining('mock data'));
+  expect(text2).toEqual(expect.stringContaining('mock data'));
+  expect(tsAfterCalls - tsBetweenCalls).toBeLessThan(tsBetweenCalls - tsBeforeCalls);
+});
+
+/* The following functions from commHelper.ts are not tested because they are just wrappers
+    around the native functions in BEMServerComm.
+  If we wanted to test them, we would need to mock the native functions in BEMServerComm, but
+    this would be of limited value. It would be better to test the native functions directly.
+
+  * - getRawEntries
+  * - getPipelineRangeTs
+  * - getPipelineCompleteTs
+  * - getMetrics
+  * - getAggregateData
+  * - registerUser
+  * - updateUser
+  * - getUser
+
+*/