test: fix vinyl/deferred_delete failure

The patch fixes the following test failure:

 | --- vinyl/deferred_delete.result        Fri Mar 15 18:17:05 2019
 | +++ vinyl/deferred_delete.reject        Fri Mar 15 18:18:18 2019
 | @@ -577,7 +577,7 @@
 |  ...
 |  sk:stat().rows -- ditto
 |  ---
 | -- 5
 | +- 25
 |  ...
 |  s:drop()
 |  ---

The failure was introduced by commit 6dd0d2fb ("vinyl: do not apply
run_count_per_level to the last level"). Due to the commit compaction of
the secondary index may happen before compaction of the primary index,
in which case deferred DELETE statements won't make it to the secondary
index in time against the test expectation. Fix this by making the first
run big enough to prevent major compaction from kicking in.

Follow-up #3657
Closes #4047