libsnapshot: update opcountcheck

Since variable block compresses blocks and there is no longer a 1:1
mapping between ops to blocks, we need to update this check in
EmitBlocks to the actual number of compressed blocks written.

Since single threaded + multi threaded + no compression invoke different
code paths. Ensure that that blocks written are still equivalent to
blocks.size(). Adding two test cases to cover these situations.

Test: th
Change-Id: If81eccf74333292a114268862dde0fe49681ef35