Prevent PHPDBG from issuing notices in unit tests

Prevent PHPDBG from issuing notices in unit tests

Although issuing PHP notices in unit tests is only a nice-to-have, the cause
is PHPDBG has no special treatment in JobQueueGroup at the contrary of the
PHP SAPI 'cli', and it may be desirable both 'command line SAPIs' have the
same behaviour.

This is the most apparent difference between cli and phpdbg but there could
be a dozen of other occurrences (see bug), potentially creating differences
in unit tests depending on the SAPI.

Bug: T162591
Change-Id: Idf9c14db72f1f768c5a17b49ed689a05922c57d3