test: fix flaky rebalancer/rebalancer.test.lua

After the previuos commit the test became pretty flaky and fails with
error "Duplicate key exists in unique index 'pk' in space '_bucket'",
when trying to force create buckets.

Let's properly wait for all buckets to be garbage collected and not just
one, as it was before.

Follow-up #351

NO_DOC=test

Author: Serpentian

test/rebalancer/rebalancer.result

@@ -360,7 +360,7 @@ test_run:switch('box_1_a')
 vshard.storage.rebalancer_disable()
 ---
 ...
-wait_bucket_is_collected(100)
+for i = 91, 100 do wait_bucket_is_collected(i) end
 ---
 ...
 vshard.storage.bucket_force_create(91, 10)

test/rebalancer/rebalancer.test.lua

@@ -181,7 +181,7 @@ util.map_bucket_protection(test_run, {REPLICASET_1}, true)
 --
 test_run:switch('box_1_a')
 vshard.storage.rebalancer_disable()
-wait_bucket_is_collected(100)
+for i = 91, 100 do wait_bucket_is_collected(i) end
 vshard.storage.bucket_force_create(91, 10)
 space = box.space.test
 space:replace{1, 91}