Update stored_map_works test, triggers potential bug.

[Mr-Leshiy]

I have updated 'stored_map_works' test case with the usage of the 'try_mutate_exist' function to trigger behavior described in this discussion #9463.
Here is the description of the test case:

1. Set up the AccountData into the '55'.
2. check that 'providers' and 'consumers' has been unchanged, 'data' is equal to '55'.
3. change data to the 'default' value '0'.
4. check that 'providers' and 'consumers' has been unchanged, 'data' is equal to '0'.
5. change data to the '55' again.
6. check that 'providers' and 'consumers' has been unchanged, 'data' is equal to '55'.

But the existing behavior is different that I am expecting to this test case, so after changing the value from the 'default' the 'providers' counter has been incremented.

[gui1117]

I think it is the expected behavior no ?

in frame-system it is specified.

/// Implement StoredMap for a simple single-item, provide-when-not-default system. This works fine
/// for storing a single item which allows the account to continue existing as long as it's not
/// empty/default.
///
/// Anything more complex will need more sophisticated logic.
impl<T: Config> StoredMap<T::AccountId, T::AccountData> for Pallet<T> {

The idea is that the storage map is considered providing when it has a value different that default

[Mr-Leshiy]

@thiolliere , I see. I will close this PR