Fix an inappropriate use of virtual methods in an execution test

One of the execution tests, UnitTestScatterAndMaskWithTopology, was defining a hierarchy of worklets that defined a virtual operator(). It mostly still worked since the code was executed with the final type and thus did not actually need to look up the virtual table. However, some invalid memory access must have been happening in CUDA since under some unpredictable circumstances a memory protection error was happening.

This fixes the problem by removing the virtual methods.

This should fix the failure of UnitTestScatterAndMapperWithTopology introduced in 35276434 (!1980 (merged)).