Rework FPI tests

[Fumuran]

What should be done?

See #896 (comment)

How should it be done?

The indexes of the obtaining values should be changed from 0 to 1 and 2 to get the value from the first slot and a map from the second.

When is this task done?

This task is done when the tests will check that we can correctly obtain elements not only from 0th storage slot.

Additional context

No response

[bobbinth]

@Fumuran - is this now closed by #957 or is there anything else outstanding?

[Fumuran]

There are some issues about FPI tests which I described in the PR, but in terms of this issue the work is done, so I think we can close it.

[Fumuran]

Closed by #957.