clean up testfiles folder

Leave only used file in the folder:
Used in the tests are:

- peptides.txt _also in extdata_
- graphs_coll_pept_prot_test.rds
- graphs_coll_prot_test.rds.rds
- edgelist_coll_pept_prot_test.rds
- edgelist_coll_prot_test.rds
- test_iterateOverCi_file1.rds
- test_iterateOverCi_file2.rds
- test_minimizeSquaredError_file1.rds
- test_minimizeSquaredError_file2.rds