libsnapshot: Don't map "b" partitions during testing.

This is testing behavior that no longer exists. We never perform an OTA
while system_other is mapped, since it would collide with the snapshot
for system.

Bug: 208944665
Test: vts_libsnapshot_test
Change-Id: I1e462fad3fd59b8b881d98da982a7c22408b837b