Add integration tests

[bvreede]

At the moment, we test era5cli in two ways:

• Test if arguments given on the command line are parsed correctly and entered correctly to the Fetch class (test_cli.py)
• Test if information given to the Fetch class result in a correct CDS query (test_fetch.py).

It would be good to also add integration tests that combine these two, where arguments are given and the final query is asserted. This would make it easier to write tests for the tool to assert correct behavior of the tool as a whole.