benchmarks: override the correct flush_threshold_mb
benchmarks: override the correct flush_threshold_mb

As of commit fb4eab5 mt-tablet-test no longer exposes flush_threshold_mb
when built statically. That's because no code paths in mt-tablet-test
invoke TabletPeer::RegisterMaintenanceOps, so the linker is able to elide
all of the symbols in tablet_peer_mm_ops.cc.

Further investigation revealed that flush_threshold_mb never had any effect
on mt-tablet-test as it doesn't use the MM. Instead, the test has its own
threads that periodically call flush and compact tablet methods, and exposes
a separate gflag (tablet_test_flush_threshold_mb) to control the flush
threshold. So let's override that gflag instead.

Change-Id: I7b373f817086065f9c6bfe846b43b0c40fa9e3cf
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5938
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>
Tested-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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