tests: bsim: gatt: ccc_store: Replace bt_gatt_write with bt_gatt_subscribe

This changes the way the GATT Client subscribes to the characteristic
value changes. The bt_gatt_write usage was replaced by dedicated
bt_gatt_subscribe API function call, so that the test verifies the GATT
Client API side as well.

Signed-off-by: Mariusz Skamra <mariusz.skamra@codecoup.pl>