Add tests in experimental off-chain env for erc721

[cmichi]

Mostly copy/paste from the existing tests.

• Got rid of push_execution_context/pop_execution_context, replaced it with set_sender instead.
• Removed all the CallData stuff, since it wasn't used anyway.
• Replaced the default_accounts implementation.

[paritytech-cicd-pr]

🦑 📈 ink! Example Contracts ‒ Changes Report 📉 🦑

These are the results when building the examples/* contracts from this branch with cargo-contract 0.17.0-638ad57 and comparing them to ink! master:

	Δ Optimized Size	Δ Used Gas	Total Optimized Size	Total Used Gas
accumulator			1.28 K
adder			2.23 K
contract-terminate			1.23 K	214_418
contract-transfer			8.37 K	14_418
delegator			6.24 K	46_504
dns			9.62 K	43_254
erc1155			27.49 K	86_508
erc20			9.09 K	43_254
erc721			14.37 K	115_344
flipper			1.55 K	14_418
incrementer			1.49 K	14_418
multisig			26.12 K	94_107
proxy			2.98 K	29_508
rand-extension			4.38 K	14_418
subber			2.24 K
trait-erc20			9.36 K	43_254
trait-flipper			1.33 K	14_418
trait-incrementer			1.42 K	28_836

Link to the run | Last update: Wed Mar 2 02:51:17 CET 2022