Add a cutoff to the size of stdout allowed in tests

Sometimes test jobs produce an unreasonable amount of stdout (we observer several GB of stdout in some misconfigured tests in LHCb).

When that happens, it is clearly a mistake and it has bad effects like ctest crashing with a bad_alloc exception or with problem in the LHCb web application that extract test results form ctest XML reports.

Similar to the timeout option, it would be extremely useful if the test wrapper GaudiPolicy/python/GaudiTesting/BaseTest.py had a cutoff so that a test is considered failed if it produces more than a few MBs of stdout.