Clean up tests for Order request

[kevinlacaille]

Move mock response and mock post out of the body of functions: #519 (comment)

Review benefit of testing the permutation of id strings in multiple tests: #519 (comment)

Invoke planet-data-request and planet-data-create in different tests: #519 (comment)
- save some order request, say something like {"name": "test_order", "products": [{"item_ids": ["20200922_183724_23_106a", "20200922_183722_17_106a"], "item_type": "PSScene4Band", "product_bundle": "analytic"}]} in a new file in a tmpdir
- Invoke create command invoke(['create', 'tests/data/newfile.json')

Combining geom_geojson and write_to_tmp_json_file into a single pytest fixture: #519 (comment)