snapuserd: Test both async and sync paths.

Rather than split this into two binaries as we did for libsnapshot, this
uses test parameterization. I went this route because even for async
builds, the sync path must always work as a fallback.

Bug: N/A
Test: snapuserd_test
Change-Id: Ib68c8fd831c345f41a4e14a99fd0a0b8b1d47b6d