make `dash_duo.take_snapshot` more flexible

Is your feature request related to a problem? Please describe.

Seems like in dash.testing, calling dash_duo.take_snapshot enforces the directory name and the naming convention:
https://github.com/plotly/dash/blob/fed4a9123f5d0cf6213da157bb94f5e1900c3440/dash/testing/browser.py#L153-L171

Describe the solution you'd like

I think that giving the option to change the path and choose the exact name of the snapshot might be nice. Maybe something like this:

def take_snapshot(self, name, save_dir="/tmp/artifacts", append_session_id=True):
    """Hook method to take snapshot when a selenium test fails. The
    snapshot is placed under.
        - `/tmp/dash_artifacts` in linux
        - `%TEMP` in windows
    with a filename combining test case name and the
    running selenium session id
    """
    target = save_dir if not self._is_windows() else os.path.join(os.getenv("TEMP"), save_dir)
    try:
        os.makedirs(target, exist_ok=True)
    except OSError:
        logger.exception("cannot make artifacts")

    if append_session_id:
        name += str(self.session_id)
    
    name += ".png"

    self.driver.save_screenshot(
        os.path.join(target, name)
    )

Start in dash/testing/browser.py at the take_snapshot entry point linked in the issue, and read how the current directory and filename are assembled. Done means callers can choose the save path and exact snapshot name, with session ID appending remaining optional and the documented platform behavior preserved.