qht-bench: relax test_start/stop atomic accesses

test_start/stop are used only as flags to loop on. Barriers are unnecessary,
since no dependent data is transferred among threads apart from the flags
themselves.

This commit relaxes the three accesses to test_start/stop that were
not yet relaxed.

Signed-off-by: Emilio G. Cota <cota@braap.org>
