Introduce PyScriptTest, an helper class to write integration tests (#663)

This PR is about integration tests: they use playwright to load HTML pages in the browser and check that PyScript works as intended, as opposed to unit tests like the ones being introduced by #665 and #661.

The main goal of this PR is to introduce some machinery to make such tests easier to write, read and maintain, with some attention to capture enough information to produce useful error messages in case they fail in the CI.

In order to use the machinery, you need to subclass tests.support.PyScriptTest, which provides several useful API calls in the form self.xxx().

See the full description here:
https://github.com/pyscript/pyscript/pull/663

Co-authored-by: Mariana Meireles <marian.meireles@gmail.com>
Co-authored-by: mariana <marianameireles@protonmail.com>

pyscriptjs/tests/test_examples.py

@@ -17,6 +17,8 @@ from urllib.parse import urljoin
 
 import pytest
 
+from .support import ROOT
+
 MAX_TEST_TIME = 30  # Number of seconds allowed for checking a testing condition
 TEST_TIME_INCREMENT = 0.25  # 1/4 second, the length of each iteration
 TEST_ITERATIONS = math.ceil(
@@ -188,6 +190,9 @@ def wait_for_render(page, selector, pattern):
 
 @pytest.mark.parametrize("example", EXAMPLES)
 def test_examples(example, http_server, page):
+    # make sure that the http server serves from the right directory
+    ROOT.join("pyscriptjs").chdir()
+
     base_url = http_server
     example_path = urljoin(base_url, TEST_PARAMS[example]["file"])