tst_QObbject: Fix very annoying -Wdeprecated-copy warnings

There's like three pages of this when compiling the test :)

Change-Id: I923f2c4f5eff7c709977026666cc5b2a2cbfaa72
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

tests/auto/corelib/kernel/qobject/tst_qobject.cpp

@@ -1401,6 +1401,7 @@ struct CustomType
     CustomType(const CustomType &other): i1(other.i1), i2(other.i2), i3(other.i3)
     { ++instanceCount; playWithObjects(); }
     ~CustomType() { --instanceCount; playWithObjects(); }
+    CustomType &operator=(const CustomType &) = default;
 
     int i1, i2, i3;
     int value() { return i1 + i2 + i3; }