Make CI check format of UI tests

[cmichi]

Closes #531.

[cmichi]

Currently blocked by rust-lang/rustfmt#4487.

Edit: I've provided rustfmt with a PR to fix the above issue, the PR has been merged in the meantime. Now we just have to wait until 1.4.23 is released.

[codecov-io]

Codecov Report

Merging #534 (b54bcce) into master (402a694) will decrease coverage by 0.62%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #534      +/-   ##
==========================================
- Coverage   67.87%   67.25%   -0.63%     
==========================================
  Files         155      155              
  Lines        6826     6895      +69     
==========================================
+ Hits         4633     4637       +4     
- Misses       2193     2258      +65     

Impacted Files	Coverage Δ
...es/lang/macro/tests/ui/pass/02-flipper-contract.rs	8.33% <0.00%> (ø)
...tes/lang/macro/tests/ui/pass/05-erc721-contract.rs	0.59% <0.00%> (-0.44%)	⬇️
crates/storage/src/collections/hashmap/mod.rs	97.54% <0.00%> (-0.81%)	⬇️
...ates/storage/src/collections/hashmap/fuzz_tests.rs	100.00% <0.00%> (+5.47%)	⬆️
crates/storage/src/collections/hashmap/impls.rs	94.11% <0.00%> (+9.90%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 402a694...b54bcce. Read the comment docs.

[cmichi]

My rustfmt PR has been merged and is in the latest release. So this PR is ready for review now.

[Robbepop]

Thanks a lot for this, @cmichi ! :)