tests: Bluetooth: ascs: Swap bt_gatt_attr_read mock with real function

This refactors the code to use exact copy of bt_gatt_attr_read instead
of mock, as the implementation might call the function with stack
allocated value. Using mock would require deep copy of the value for
further verification. As the buffer where the value is about to placed
is one of the attr->read() parameters, the test may call the read()
callback with stack allocated buffer of defined size and read the value
from the buffer.

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