tests/cpydiff/core_import_prereg: Fill in cause and workaround.

tests/cpydiff/core_import_prereg.py

 """
 categories: Core,import
 description: Failed to load modules are still registered as loaded
-cause: Unknown
-workaround: Unknown
+cause: To make module handling more efficient, it's not wrapped with exception handling.
+workaround: Test modules before production use; during development, use ``del sys.modules["name"]``, or just soft or hard reset the board.
 """
 import sys