test: fix hang in storage/recovery.test

The problem is that recovery fiber wakes up earlier than we want it
to do so. This leads to the test output which we don't expect.

Let's block recovery fiber before making any changes to the `_bucket`.
It'll start again as soon as the instance is restarted.

Needed for tarantool#341

test/storage/recovery.result

@@ -122,6 +122,9 @@ while _bucket:count() ~= 2 do vshard.storage.recovery_wakeup() fiber.sleep(0.1)
 _ = test_run:switch('storage_1_a')
 ---
 ...
+vshard.storage.internal.errinj.ERRINJ_RECOVERY_PAUSE = true
+---
+...
 _bucket:replace{1, vshard.consts.BUCKET.SENDING, util.replicasets[2]}
 ---
 - [1, 'sending', '<replicaset_2>']

test/storage/recovery.test.lua

@@ -58,6 +58,7 @@ while _bucket:count() ~= 2 do vshard.storage.recovery_wakeup() fiber.sleep(0.1)
 -- must restore buckets, when the destination is up.
 --
 _ = test_run:switch('storage_1_a')
+vshard.storage.internal.errinj.ERRINJ_RECOVERY_PAUSE = true
 _bucket:replace{1, vshard.consts.BUCKET.SENDING, util.replicasets[2]}
 _ = test_run:switch('storage_2_a')
 _bucket:replace{1, vshard.consts.BUCKET.ACTIVE}