Fixes#6195
test_commitlog_delete_when_over_disk_limit reads current segment list
in flush handler, to compare with result after allowing deletetion of
segement. However, it might be called more than once in rare cases,
because timing and us using rather small sizes.
Reading the list the second time however is not a good idea, because
it might just very well be exactly the same as what we read in the
test check code, and we actually overwrite the list we want to
check against. Because callback is on timer. And test is not.
Message-Id: <20200414114322.13268-1-calle@scylladb.com>
a62d75fed5