Silence qqmlecmascript::propertyVarInheritance failures with clang release builds

There's no equivalent to #pragma GCC optimize ("O0") yet, so we cannot run
these test (sort of) reliably with clang.

Change-Id: Ie41d3ba7911cefacca00165941761b20579107a2
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>