Update the tests to use existing stacks

[cmarshak]

Background

Pytests hit the ASF API on each test to generate stack. In dense frame areas, can take nearly 1 minute. Then there are are
18 different tests.

Solution

Test this function so we don't have to generate stack for each test as is currently done testing this function.

Also put the month filter into the former function to ensure cleaner and more organized code.

The tests will have geojson stacks for these tests.