liblp: Add unit tests for flashing and readback.

These tests check that various aspects of liblp's on-disk management
are functioning as intended. This checks redundancy, metadata slot
management, and metadata update and readback.

Bug: 79173901
Test: liblp_test gtest
Change-Id: Ib780676e0f34f44aa255e8fcfded2ceb71fe3dce