updated testcases and added testfile

[ArthurDeclercq]

This pull request includes changes to the tests/test_mumble.py file and the addition of a new test data file. The most important changes include adding a new test data file, updating imports, and modifying test cases to use the new data file.

Major changes

• primarily addition of packaging data, that includes default ptm list and unimod database.
• Add class method for removing cached ptm database

Test Data Addition:

• tests/test_data/unmapped_mass_shift_psms.tsv: Added a new test data file containing PSM information.

Test Case Modifications:

• tests/test_mumble.py: Updated imports to include read_file from psm_utils.io and removed unused imports.
• tests/test_mumble.py: Modified the test_single_combined_modifcations and test_double_combined_modifcations test cases to use the new test data file and validate the expected peptidoforms and the number of mapped PSMs.