Update dApp tests to support the single-asset balance model.

[Shaptic]

After #80, the tests still need to be updated.

Based on Discord threads (1 and 2), we need to manually create a LedgerSnapshot instance containing the xdr::TrustLineEntrys to the token admin for each of the participating accounts.

The following code gets part of the way there, but it relies on importing stellar_xdr as a separate crate, which conflicts with stellar_sdk::xdr, meaning the trustline entries can't actually be added to the entries vector due to differing types:

        // where `fxdr` comes from:
        //
        //    extern crate xdr as full_xdr;
        //    use full_xdr::next as fxdr;

        let token_admin = e.accounts().generate_and_create();

        let snap = e.to_snapshot();
        let entries = &snap.ledger_entries;

        for trustor in &std::vec![recipient, user1, user2] {
            let token_issuer = fxdr::AccountId(
                match TryInto::<xdr::AccountId>::try_into(token_admin).unwrap().0 {
                    xdr::PublicKey::PublicKeyTypeEd25519(pk) => {
                        fxdr::PublicKey::PublicKeyTypeEd25519(fxdr::Uint256(pk.0))
                    }
                },
            );

            let account_id = match TryInto::<xdr::AccountId>::try_into(trustor).unwrap().0 {
                xdr::PublicKey::PublicKeyTypeEd25519(pk) => {
                    fxdr::PublicKey::PublicKeyTypeEd25519(fxdr::Uint256(pk.0))
                }
            };

            let asset = fxdr::TrustLineAsset::CreditAlphanum4(fxdr::AlphaNum4 {
                asset_code: fxdr::AssetCode4([0x41; 4]),
                issuer: token_issuer,
            });

            let trustline = fxdr::LedgerKey::Trustline(fxdr::LedgerKeyTrustLine {
                account_id: fxdr::AccountId(account_id),
                asset,
            });

            let trustline_entry = (Box::new(key), Box::new(trustline));
            entries.push(trustline_entry);
        }

        snap.update_entries(&entries);
        e = soroban_sdk::Env::from_snapshot(snap);

Once stellar_sdk exposes all XDR types (or provides a convenient way to make trustlines), this can be modified to work.

[paulbellamy]

Once fixed, we need to revert commit 56f5bef

[esteblock]

This issue should be marked as Closed @paulbellamy