String: Split memcpy tests so that parallel build is faster

No bug.

This commit splits test-memcpy.c into test-memcpy.c and
test-memcpy-large.c. The idea is parallel builds will be able to run
both in parallel speeding up the process.

Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>