[murugapy] Adding tests

[prasithbasky]

As required by the excercise the following is completed

1. Adding assertion statements
2. writing unit tests in pytest
3. writing integration tests
4. checking test coverage
5. creating tox for automation tests

the results of tests:

basky@Basky:~/SSE/testing-python-exercise$ python3 -m pytest
================================================= test session starts ==================================================
platform linux -- Python 3.12.3, pytest-7.4.4, pluggy-1.4.0
rootdir: /home/basky/SSE/testing-python-exercise
collected 5 items

tests/integration/test_diffusion2d.py ..                                                                         [ 40%]
tests/unit/test_diffusion2d_functions.py ...                                                                     [100%]

================================================== 5 passed in 0.27s ===================================================