Factor out the run-in-a-temporar-directory pytest fixture into conftest.py

[mwcraig]

Right now there are several places (and one more on the way) where we repeat this block of code:

    # This auto-used fixture changes the working directory to the temporary directory
    # and then changes back to the original directory after the test is done.
    @pytest.fixture(autouse=True)
    def change_to_tmp_dir(self, tmp_path):
        original_dir = os.getcwd()
        os.chdir(tmp_path)
        # Yielding here is important. It means that when the test is done, the remainder
        # of the function will be executed. This is important because the test is run in
        # a temporary directory and we want to change back to the original directory
        # when the test is done.
        yield
        os.chdir(original_dir)

We should move this to conftest.py (but will need to take out the autouse=True maybe). In places we are currently using it with autouse=True we will to keep a small fixture in place to specify the autouse=True part.

If we take out the autouse thing in conftest then using the fixture would look like this (I think this is untested):

@pytest.fixture(autouse=true)
def run_in_temp_dir(self, change_to_tmp_dir):
    pass