mem: Correct testcases

mem: Correct testcases

Currently the 'mem' module testcases use test/run.c even though they don't
rely on access to the module internals.  They're also missing an include
of mem.c, which has the effect that on systems with a C library memmem()
implementaiton, only that is tested, not the (re-)implementation in the
mem module itself.  This corrects that by moving run.c to api.c/

Additionally, the memmem() testcases don't cover the case where the
"needle" appears multiple times in the "haystack".  This patch also adds
such a test.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>