tests/lvm: re-introduce POOL_CONF
tests/lvm: re-introduce POOL_CONF

It's still needed by the lvm integration tests. If possible, it should
be avoided inside lvm unit tests though as using it will break
compatibility with callback unit tests.

marmarek/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin