Skip selenium e2e helpers at collection when their deps are missing (#24)

streamlitfront/tests/conftest.py

@@ -1,3 +1,31 @@
+"""Pytest configuration for the in-package tests.
+
+``common.py`` and ``use_case_test_old.py`` drive a real browser through selenium; they
+cannot even be imported without the optional e2e dependencies (``tests/test_requirements.txt``),
+so they are left out of collection when those are missing instead of breaking it.
+"""
+
+from importlib.util import find_spec
+
+
+def _importable(name: str) -> bool:
+    try:
+        return find_spec(name) is not None
+    except ImportError:  # a parent package is missing
+        return False
+
+
+_E2E_MODULES = ("selenium", "webdriver_manager", "strand")
+_E2E_FILES = ("common.py", "use_case_test_old.py")
+
+collect_ignore = (
+    []
+    if all(map(_importable, _E2E_MODULES))
+    and _importable("front.tests.test_use_case")
+    else list(_E2E_FILES)
+)
+
+
 def pytest_addoption(parser):
     parser.addoption("--headless", action="store_true", default=False)