Fix unit tests for Mac.
Fix unit tests for Mac.

Was generating signed/unsigned warnings under Clang.

Tested: Unit tests build and run fine on Mac now
Change-Id: I1cf0c9dd69db18f240859e1b53c3c2092756ed11

google/fplutil,google/fplutil,google/fplutil,google/fplutil,google/fplutil,google/fplutil