Enable test coverage

[piobab]

no space left for test coverage in red-bank repo.

Problem:
Tests are structured:

tests
    test_deposit.rs
    test_borrow.rs
    test_withdraw.rs
    ...

Every test_ file gets treated as a separate crate.
Generating coverage requires to enable -Cinstrument-coverage flag which is used for report generation. It generates instrumented binary files for each test module (crate). These files can be quite large. It consumes almost 2x more space than normal cargo make test.

Solutions:

1. Increase github worker disk space
2. Run test cases one by one for every contract and cleanup used space after each run. Increase coverage job time (it is already quite long).
3. Merge few seperate test_ files (crates) to one.
   before my PR target/coverage ~ 6,5 GB:

outposts on  fix/MP-3258-coverage [$] via 𝗥 v1.69.0
➜ du -sh target/*                       
4,0K    target/CACHEDIR.TAG
6,5G    target/coverage
122M    target/release
590M    target/wasm32-unknown-unknown

after my PR target/coverage ~ 4,3 GB (2,2 GB reduction):

outposts on  fix/MP-3258-coverage [$] via 𝗥 v1.69.0
➜ du -sh target/*
4,0K    target/CACHEDIR.TAG
4,3G    target/coverage
122M    target/release
  0B    target/tmp
591M    target/wasm32-unknown-unknown

We chose solution nr 3. End goal is to simplify these tests and move some of them (unit tests) close to original function (src) and in tests keeps only integration tests (multitest).

[piobab]

Resolved conflicts and removed unused comments.