[Backport 11.2] Provide (log4j1) Log4jInterceptor test utility class to suppress and/or capture log events during unit tests

To support the REPO-1879 backport, the current v12 log4j2 Log4jInterceptor test utility class from CMS-10708 also will be backported to v11.2.

The target package and API for this utility will kept equal, but of course the implementation will have to be adapted to log4j1 instead of log4j2.

As this utility currently only is needed to support the backport of REPO-1879, the backport will be done into the repository testutils module.

If future backport needs require this utility to be more generically available, then a backport of the hippo-cms-testutils project itself may be needed, and then this utility can be moved there.