health-check: Fixing stupid syntax error
health-check: Adding forked mem performance

When trying to estimate the memory performance, the former approach was
to run 8 threads inside sysbench.

In fact, the measured bandwidth vary a lot when running 8 forks of
sysbench running a single thread. It seems some optimization occurs
while running threads.

So this patch implements the forked version while previous code was
featuring the threaded one.

Here come a typical output:
('cpu', 'logical', 'threaded_bandwidth_1K', '1149'),
('cpu', 'logical', 'threaded_bandwidth_4K', '3227'),
('cpu', 'logical', 'threaded_bandwidth_1M', '20795'),
('cpu', 'logical', 'threaded_bandwidth_16M', '19186'),
('cpu', 'logical', 'threaded_bandwidth_128M', '13868'),
('cpu', 'logical', 'threaded_bandwidth_1G', '6654'),
('cpu', 'logical', 'forked_bandwidth_1K', 16517),
('cpu', 'logical', 'forked_bandwidth_4K', 39194),
('cpu', 'logical', 'forked_bandwidth_1M', 60867),
('cpu', 'logical', 'forked_bandwidth_16M', 7162),
('cpu', 'logical', 'forked_bandwidth_128M', 7173),
('cpu', 'logical', 'forked_bandwidth_1G', 7190),