libsnapshot: Add a test case for recent decompress regression.

The attached test data is a 4096 length byte run that gz compresses to
exactly 4096 bytes. This exposes an edge case in CowReader::ReadData
with v2 snapshot files.

Bug: 310191184
Test: cow_api_test
Change-Id: I35e8d7e939d607d1dc118285ebc2f636c2291a20