liblp: Make it easier to test UpdatePartitionTable.

This change makes the internal UpdatePartitionTable function more
testable by parameterizing its write functions. It also adds two tests,
one of which exposes a flaw in the current implementation.

Bug: 79173901
Test: liblp_test gtest
Change-Id: I3c4112794b97d577a27f035baeac2d42ac75f552