test: fix vinyl/errinj failure

The patch fixes the following test failure:

 | --- vinyl/errinj.result Fri Mar 15 19:19:19 2019
 | +++ vinyl/errinj.reject Fri Mar 15 19:21:37 2019
 | @@ -854,7 +854,8 @@
 |  ...
 |  ret
 |  ---
 | -- - [3, 3]
 | +- - [2, 2]
 | +  - [3, 3]
 |  ...
 |  s:drop()
 |  ---

The error occurrs, because the test doesn't make sure that a read from
the scondary index actually occurs and the fiber does stall on a read
from the primary index. Use index stats to assure that.

Closes #3863