Signed/unsigned comparison warning fix in unit test.
Signed/unsigned comparison warning fix in unit test.

halfflat/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor,eth-cscs/nestmc-proto,halfflat/arbor