Add system test utility package isctest

Create a utility package for code shared by the python tests. The
utility functions should use reasonable defaults and be split up into
modules according to their functionality.

Ensure assert rewriting is enabled for the modules to get the most
useful output from pytest.

dangerfile.py

@@ -462,6 +462,7 @@ for testname in testnames:
         not os.path.isdir(dirpath)
         or testname.startswith(".")
         or testname.startswith("_")
+        or testname == "isctest"
     ):
         continue
     if "_" in testname: