test/mempool: fix false positive result

The ret variable, initialized to -1, is changed to 0 during the test,
making the test successful in some cases where it should return a
failure.

Fix this by always using the GOTO_ERR() macro that sets the ret
variable before doing the goto.

Fixes: 923ceaeac140 ("test/mempool: add unit test cases")

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <arybchenko@solarflare.com>