Add support for E2E (end-to-end) tests (#625)

This changeset lays the foundation for supporting E2E (end-to-end) integration tests for US Notify.  It brings in the Playwright testing framework along with the Playwright pytest plugin to make this possible, and includes the following adjustments:

- A new test session fixture for ensuring that Playwright authenticates with the sites that are currently behind HTTP Auth (requies env-var config)
- A new end_to_end test directory specifically for E2E tests
- Updates to the Makefile that make sure E2E tests are not run as a part of the normal test routine but can be run separately
- A new command in the Makefile to run E2E tests that will run in Chromium, Firefox, and Webkit headless browsers

Signed-off-by: Carlo Costino <carlo.costino@gsa.gov>

Pipfile

@@ -37,6 +37,7 @@ newrelic = "*"
 flask-talisman = "*"
 notifications-utils = {editable = true, ref = "main", git = "https://github.com/GSA/notifications-utils.git"}
 coverage = "*"
+pytest-playwright = "*"
 
 [dev-packages]
 isort = "==5.12.0"