test: disable flaky performance test

Disabled wal_off/iterator_lt_gt.test.lua test due to performance
test need to be reorganized into separate mode at the standalone
host. Currently this test doesn't show any issue, but breaks the
testing some time, with errors like:

[010] wal_off/iterator_lt_gt.test.lua                                 [ fail ]
[010]
[010] Test failed! Result content mismatch:
[010] --- wal_off/iterator_lt_gt.result	Fri Apr 12 10:30:43 2019
[010] +++ wal_off/iterator_lt_gt.reject	Fri Apr 12 10:36:30 2019
[010] @@ -79,7 +79,9 @@
[010]  ...
[010]  too_longs
[010]  ---
[010] -- []
[010] +- - 'Some of the iterators takes too long to position: 0.074278'
[010] +  - 'Some of the iterators takes too long to position: 0.11786'
[010] +  - 'Some of the iterators takes too long to position: 0.053848'
[010]  ...
[010]  s:drop()
[010]  ---
[010]
[010] Last 15 lines of Tarantool Log file [Instance "wal"][/tarantool/test/var/010_wal_off/wal.log]:

See #2539

test/wal_off/suite.ini

@@ -5,3 +5,4 @@ description = tarantool/box, wal_mode = none
 use_unix_sockets = True
 is_parallel = True
 pretest_clean = True
+disabled = iterator_lt_gt.test.lua