Create a pytest fixture in the iqtree2/tests directory to provide core testing services

[khiron]

conftest.py file to set up the environment for all tests

• access to the binary iqtree2 file
• creation of a temp directory for each test
• removal of the temporary after each test
• setting the working folder for the duration of the test as that temp directory
• create a /data subdirectory of that temp directory
• the ability for tests to list the data files (from /tests/data) that will be required and copying those into the /data path