MDLSITE-2096 Detect unit test files having multiple classes.

By default ($multipleclassiserror = empty) the check is just informative, sending some WARNINGs to output. If that setting is enabled (=yes), then any warning causes the job to fail.

Note that the 2 CI servers have been already configured to add the new param (defaulting to blank) for the master jobs.

This comes from https://tracker.moodle.org/browse/MDLSITE-2096

Ciao :-)