Implement unit tests for remaining string library functions. Fix memory leak in test_strdup(). Signed-off-by: Anna Karas <aka@semihalf.com> Change-Id: I8ac6a6b2413d9077dc9ea81f638a2b0acd5c8862 Reviewed-on: https://review.coreboot.org/c/coreboot/+/42311 Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org> Reviewed-by: Julius Werner <jwerner@chromium.org> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>