benchtests: Use TEST_NAME instead of hardcoding memcpy

The hardcoded 'memcpy' name turns up in other derived tests like
mempcpy.

       * benchtests/bench-memcpy.c (test_main): Use TEST_NAME instead of
       hardcoding memcpy.
       * benchtests/bench-memcpy-large.c (test_name): Likewise.
       * benchtests/bench-memcpy-random.c (test_name): Likewise.