Add tests for normalize_date_range function when querying DB

[Kate Manning]

if/when we keep using that, we will need to add tests for this function.

Originally posted by @karlcow in #93

[Kate Manning]

This was actually done by @karlcow in PR #108:

webcompat-metrics-server/tests/unit/test_helpers.py

Line 112 in 361afd8

 def test_normalize_date_range(self):