benchtests: Add more tests for memrchr

benchtests: Add more tests for memrchr

bench-memchr.c is shared with bench-memrchr.c.  This patch adds some
tests for positions close to the beginning for memrchr, which are
equivalent to positions close to the end for memchr.

	* benchtests/bench-memchr.c (do_test): Print out both length
	and position.
	(test_main): Also test the position close to the beginning for
	memrchr.