Bluetooth: Mesh: Fix heartbeat subscription tests

MESH/NODE/CFG/HBS/BV-01-C expects the MinHops to be 0x7f after
disabling subscription, but 0x00 for subsequent Get requests.

MESH/NODE/CFG/HBS/BV-02-C expects us to return previous
count value and then reset it to 0.

Signed-off-by: Michał Narajowski <michal.narajowski@codecoup.pl>