tests: bsim: Bluetooth: Fix GATT long read test

The test where it was supposed to read the long characteristic,
only read the "small" characteristic, due to setting the wrong handle.

With the right handle, the test failed and the test had to be
updated to store the data across multiple reads, and then do
the proper comparison.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>