Write unit tests for upstream processing script

Now that scripts/prepare_inputs.py (soon to be renamed zarp.py) shapes up to be the main entry point for users running ZARP and gets longer and longer, apart from packaging it properly (see #135 (closed)), it becomes crucial to unit test all parts of it.

• parse_cli_args()
• setup_logging()
• fetch_labkey_table()
• get_read_length()
• kmer_from_read_length()
• get_strand_param_kallisto()
• get_strand_param_alfa()
• get_strand_names_alfa()
• get_polya_adapter_seqs()
• expand_path()
• main()