Fix Broken Test for firebaseTsFromISO() in time.js

[zghera]

Issue summary

Because Timestamp was mocked in time.test.js the new test for firebaseTsFromISO fails because the constructor for Timestamp is not mocked. So constructor either needs to also be mocked or a workaround for using the real constructor in this test should be found (maybe something like this).

Expected behavior

Test firestore Timestamp format test in time.test.js passes.

Actual behavior

Test firestore Timestamp format test in time.test.js fails.