Add tests in experimental off-chain env for multisig

[cmichi]

Mostly copy/paste with some adaptions for the new API.

[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 03:25:52 CET 2022

[codecov-commenter]

Codecov Report

Merging #1159 (7056d19) into master (39f3935) will decrease coverage by 0.15%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1159      +/-   ##
==========================================
- Coverage   62.61%   62.45%   -0.16%     
==========================================
  Files         244      245       +1     
  Lines        9298     9315      +17     
==========================================
- Hits         5822     5818       -4     
- Misses       3476     3497      +21     

Impacted Files	Coverage Δ
...env/src/engine/experimental_off_chain/call_data.rs	0.00% <0.00%> (ø)
...rates/env/src/engine/experimental_off_chain/mod.rs	100.00% <ø> (ø)
.../env/src/engine/experimental_off_chain/test_api.rs	83.67% <ø> (ø)
...ates/storage/src/collections/hashmap/fuzz_tests.rs	95.74% <0.00%> (-4.26%)	⬇️

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