chore: add test coverage command to Makefile

[fpgmaas]

Description

This PR proposes to add a make test-cov command to the Makefile to make it easier for contributors to check test coverage.

Output currently looks as follows:

[...]
tests/test_writerproperties.py::test_write_with_writerproperties PASSED    

---------- coverage: platform darwin, python 3.11.2-final-0 ----------
Name                        Stmts   Miss Branch BrPart  Cover
-------------------------------------------------------------
deltalake/__init__.py          11      0      0      0   100%
deltalake/_util.py             16      1     12      1    93%
deltalake/data_catalog.py       6      0      0      0   100%
deltalake/exceptions.py         5      0      0      0   100%
deltalake/fs.py                44     11      2      0    76%
deltalake/schema.py            60      0     22      0   100%
deltalake/table.py            431     42    174     23    89%
deltalake/writer.py           267     98    175     14    58%
-------------------------------------------------------------
TOTAL                         840    152    385     38    78%
Coverage HTML written to dir htmlcov

Also removed --cov=deltalake from the pytest ini commands, because I think that is better moved to [tool.coverage.run].

Related Issue(s)

Documentation

[ion-elgreco]

Thanks! @fpgmaas