tests/misc-lfring.cc: Delete the test for ring_mpsc.

We are about to kill ring_mpsc - delete the corresponding tests.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
ring_spsc/mpsc: Fix pop() method failing after the ring index wraps around

- Fixed ring_spsc: "empty" check should be "_beg == _end" and not "_beg >= _end".
- Deleted ring_mpsc since it's not currently used anywhere.

Fixes #225.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tests: added a test verifying the fix for issue #225

Running 'make check' after the fix:
...
TEST tst-ring-spsc-wraparound.so OK  (8.817 s)
...

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>