test: fix vinyl/errinj_stat failure

The patch fixes the following test failures:

 | [022] --- vinyl/errinj_stat.result	Tue Mar 19 17:52:48 2019
 | [022] +++ vinyl/errinj_stat.reject	Wed Mar 20 08:08:41 2019
 | [022] @@ -229,7 +229,7 @@
 | [022] ...
 | [022] stat.tasks_inprogress == 0
 | [022] ---
 | [022] -- true
 | [022] +- false
 | [022] ...
 | [022] stat.tasks_completed == 1
 | [022] ---

 | [013] --- vinyl/errinj_stat.result	Tue Mar 19 17:52:48 2019
 | [013] +++ vinyl/errinj_stat.reject	Wed Mar 20 08:11:15 2019
 | [013] @@ -168,7 +168,7 @@
 | [013] ...
 | [013] stat.tasks_inprogress > 0
 | [013] ---
 | [013] -- true
 | [013] +- false
 | [013] ...
 | [013] stat.tasks_completed == 0
 | [013] ---
 | [013] @@ -183,7 +183,7 @@
 | [013] ...
 | [013] box.stat.vinyl().scheduler.tasks_inprogress > 0
 | [013] ---
 | [013] -- true
 | [013] +- false
 | [013] ...
 | [013] errinj.set('ERRINJ_VY_RUN_WRITE_DELAY', false)
 | [013] ---

The problem occurred, because the test didn't make sure that an
asynchronous dump/compaction task has actually started/completed.
Even box.snapshot() doesn't guarantee that a dump task is complete,
in fact. This patch adds wait_cond's to guarantee the test never
fails like that anymore.

Closes #4059
Closes #4060