test: app/fiber.test.lua flaky fails

[Totktonada]

The test case 'test6' that was commited in 48cad35 fails like so from time to time:

[016] Test failed! Result content mismatch:
[016] --- app/fiber.result	Sun Dec  2 18:59:30 2018
[016] +++ fiber.reject	Sun Dec  2 19:40:28 2018
[016] @@ -1144,7 +1144,7 @@
[016]  ...
[016]  f:status();
[016]  ---
[016] -- dead
[016] +- suspended
[016]  ...
[016]  -- test side fiber in transaction
[016]  s = box.schema.space.create("test");

How to reproduce:

TEST_RUN_TESTS=$(for i in $(seq 1 100); do echo -n "app/fiber.test.lua "; done) taskset 0x2 make test (note the taskset!)

Also reproduced by Travis-CI: https://travis-ci.org/tarantool/tarantool/jobs/461369575