test: Add genome fasta file to test_data directory

As spotted by @mzavolan in the Aptardi PR review (#174), we don't currently have a genome FASTA file in the `tests/test_data` repository. This would also be useful for LABRAT and possibly other tools too.

This should be the same genome version used for alignment and be subsetted to the two chromosomes (chr6 and 16) that are present in the current test data. If the file is too large even with just two chromosomes, we should consider gzip-ing the file and shipping that with the repo, providing a note in the README to tell the downstream user to unzip if the tool requires uncompressed FASTA.

It would also be useful to generate two versions of the test file with the `chr` chromosome name prefix resent/absent for compatibility with the existing `noChr` test files and the prospective test files with the `chr` prefix added (#177).

@mrgazzara - Can you confirm the genome versions used please? From the other test file names I suspect GRCm38/mm10