Run pytraj tests

pytraj uses pytest for testing.

Quick guide

cd tests
# full test
pytest -vs .

# run test for a given file
pytest -vs test_io/test_load_state.py

# run test for a given function
pytest -vs test_io/test_write_pdb.py::test_trajectory_writer

# run test with pattern. e.g: run all tests having "frame" in their function names
pytest -vs -k frame

# run test with code coverage report
pytest -vs --cov=pytraj --cov-report=html