docs: clarify lifetime of TestCase objects

Author: ytausch

tests_integration/lib/test_case.py

@@ -8,6 +8,10 @@
 class TestCase(ABC):
     """
     Abstract base class for a single test case in a scenario.
+    Per test case, there is exactly one instance of this class statically created
+    in the definition of the ALL_TEST_CASES list of the feedstock module.
+    Note that a test case (i.e. an instance of this class) might be run multiple times,
+    so be careful with state you keep in the instance.
     """
 
     def get_router(self) -> APIRouter: