Make NeedleFinderTest::Needles256 actually do things
Make NeedleFinderTest::Needles256 actually do things

Summary:
The min value of a `char` is `-128`, but we're initializing our `size_t` i's with it, so, as MSVC quite rightly warns (as C6294) the condition is never true and the loop never loops.
Fix it by signing our i's.

Reviewed By: yfeldblum

Differential Revision: D4145419

fbshipit-source-id: 87591d0f84c54472b1b5847e025d091de3f98bca

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