Reduce 'module not found' console logging

Many users will only have the ability to resolve module 'firebase' or 'firebase-admin' but not necessarily both. 

Impacted users see repeated logging of 'Module not found, mocking skipped' messages which can cause concern, and also pollute the logs of larger test suites with many such lines.

This change only logs to console in the more severe case where neither module can be found.

src/mocks/firebase.js

@@ -39,17 +39,22 @@ const firebaseStub = (overrides, options = defaultOptions) => {
 };
 
 const mockFirebase = (overrides = {}, options = defaultOptions) => {
-  mockModuleIfFound('firebase', overrides, options);
-  mockModuleIfFound('firebase-admin', overrides, options);
+  const moduleFound = 
+    mockModuleIfFound('firebase', overrides, options) |
+    mockModuleIfFound('firebase-admin', overrides, options);
+
+  if (!moduleFound) {
+    console.info(`Neither 'firebase' nor 'firebase-admin' modules found, mocking skipped.`);
+  }
 };
 
 function mockModuleIfFound(moduleName, overrides, options) {
   try {
     require.resolve(moduleName);
     jest.doMock(moduleName, () => firebaseStub(overrides, options));
+    return true;
   } catch (e) {
-    // eslint-disable-next-line no-console
-    console.info(`Module ${moduleName} not found, mocking skipped.`);
+    return false;
   }
 }