Fix vinyl/compact test

The vinyl/compact issues two snapshots and expects that there will be
two runs after it, then it keeps waiting until compaction merges them.
There is a race condition intrinsic to this test - compaction might have
finished before the test checks that there are exactly two runs. To
eliminate it, let's set the test index's compact_wm parameter to 3 and
add one more snapshot to trigger compaction.

Since currently, it's impossible to modify vinyl options dynamically,
this patch moves compact_wm to index options where page_size and
range_size reside.

Closes #1758