Move the flow test data set header
The header file GenerateTestDataSets.h was located in
vtkm/worklet/testing. However, the only tests that used this header
are in vtkm/filter/flow/testing. Thus, the header file is moved to where
it is used.
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The header file GenerateTestDataSets.h was located in
vtkm/worklet/testing. However, the only tests that used this header
are in vtkm/filter/flow/testing. Thus, the header file is moved to where
it is used.