[Unix] Create a framework for certbot integration tests: PART 4 (#6958)

This PR is the part 4 to implement #6541. It adds the integration tests for the nginx certbot plugin, and corresponds to the certbot-ci translation of certbot-nginx/tests/boulder-integration.sh that is executed for each PR.

As with certbot core tests, tests are written in Python, and executed by pytest, against a dynamic Boulder/Pebble instance setup. Tests are parallelized, of course, and a specific IntegrationTestsContext class, extended the one from certbot core tests, is crafter for these specific tests: its main goal is to setup a specific nginx instance for the current test.

On top of that, I use the test parametrization feature of Pytest, to drastically reduce the size of the actual code: indeed, the 6 tests from the original bash script share the same logic. So using a parametrization, one unique test is written, that is then executed 6 times against 6 different sets of parameters.

Note that the module integration_tests.nginx_tests.nginx_config do the same, but in Python, than certbot-nginx/tests/boulder-integration.conf.sh. The latter will be removed in a future PR, with all other bash scripts.

* Add nginx tests

* Distribute the other_port

* Load a pre-generated key/cert for nginx config

* Correct preload, remove a test, simplify a variable

* Integrate assertion directly in the test function

* Check process is not terminated

* Add spaces in the nginx config

* Add comments

* Use indirection

* Allow external cert

* Add coverage threshold for certbot-nginx

tox.ini

@@ -260,5 +260,6 @@ commands =
         --cov-config={toxinidir}/certbot-ci/certbot_integration_tests/.coveragerc \
         -W 'ignore:Unverified HTTPS request'
     coverage report --include 'certbot/*' --show-missing --fail-under=64
+    coverage report --include 'certbot-nginx/*' --show-missing --fail-under=74
 passenv =
     DOCKER_*