Fix tests with Qt 5.14 by ignoring -Wdeprecated-declarations

Filters out these warnings, which we don't care about:

qstring-allocations/main.cpp:217:20: warning: 'sprintf' is deprecated: Use asprintf(), arg() or QTextStream instead [-Wdeprecated-declarations]
detaching-member/main.cpp:105:5: warning: 'qSort<QList<T *>::iterator>' is deprecated: Use std::sort [-Wdeprecated-declarations]

CCMAIL: dfaure@kde.org