Make CaptureLog::withMessage() raise tut::failure if not found.

All known callers were using ensure(! withMessage(...).empty()). Centralize
that logic. Make failure message report the string being sought and the log
messages in which it wasn't found.
In case someone does want to permit the search to fail, add an optional
'required' parameter, default true.
Leverage new functionality in llprocess_test.cpp.