Add tests for core functionality

[robberwick]

Adds first cut at a test suite for the SerialTransfer and CRC classes. It only provides ~85% coverage in total (~90% for CRC.py and 84% for pySerialTransfer.py). In part this is due to not covering the non core library functionality i.e. the crc __name__ == "__main__" code and the helper functions such as msb, lsb etc.

The coverage is not the full story though, obviously. There are definitely untested code paths here also (which can be seen if pytest is run with coverage turned on), so there are definitely areas for improvement. This feels like a good start however, and I think will go some way to providing a defence against regressions when extending or refactoring the functionality of the library.

It's also worth noting that the content of the tests themselves probably wants reviewing to double check that the things i am asserting in the tests are actually correct, as the'll be treated as the source of truth. As you did all the excellent work in the first plaec, you'll be well placed to point out where i've made stupid mistakes :-).

Note: I have added pytest, pytest-mock, and pytest-cov as dev dependencies. In order to run the tests, it is necessary to perform an editable install of the package, passing the extra [dev] qualifier e.g. pip install -e .[dev]. Tests can then be run with just pytest, or with coverage enabled with pytest --cov .\pySerialTransfer

Addresses #89

[PowerBroker2]

This is a dumb question, but did you run the tests and everything checked out?

[robberwick]

that's not a dumb question, that's a very good question. I should have included the output of a test run in the pr description.

========================================== test session starts ===========================================
platform win32 -- Python 3.11.0, pytest-8.1.1, pluggy-1.4.0
plugins: cov-5.0.0, mock-3.14.0
collected 80 items

tests\test_crc.py .............x..x..                                                               [ 23%]
tests\test_py_serial_transfer.py .....................................x................x......      [100%]

---------- coverage: platform win32, python 3.11.0-final-0 -----------
Name                                   Stmts   Miss  Cover
----------------------------------------------------------
pySerialTransfer\CRC.py                   42      4    90%
pySerialTransfer\__init__.py               1      0   100%
pySerialTransfer\pySerialTransfer.py     303     42    86%
----------------------------------------------------------
TOTAL                                    346     46    87%

===================================== 76 passed, 4 xfailed in 0.73s ======================================

x denotes an expected fail: test_calculate_with_dist_greater_than_list_length test_calculate_with_negative_dist

• they're both skipped as the calculate method currently fails those tests. once the tests are merged, i'll raise an issue (or just a pr) to enable the tests and fix the edge cases.

test_tx_obj_known_types with a val_type_override of 'c' doesn't gracefully handle exceptions. not sure what to do here, so marked the test as xfail test_tick_with_valid_data_and_non_callable_callback - I raised an issue for this, and have a pr ready to go, but it of course relies on having the test suite, and i wanted to keep this pr focused on only introducing tests.

[PowerBroker2]

Ok, thanks!