Fix get_asset_chunk unit test

The root cause of recent build failures on our CI system is a test that relies on some content already present in the DB that supports our testcase. 

I remark that the "functional" part of the code has already been extracted and unit tested properly.

The test that fails is actually doing an I/O operation which used to rely on the DB content. This pull request makes this test just check that a proper query is made and that the returned value is a container with an expected number of items.