test: refactor unit tests with solc downloader

[antonbaliasnikov]

What ❔

• [x] Moved src/tests to tests folder for more canonical tests organization accordingly to the Rustbook
• [x] Add Downloader module in tests that downloads all required solc versions on-the-fly
• [x] Use zksolc with help of assert_cmd crate instead of requirement of adding it to the PATH
• [x] Small refactoring and unification of testing code

Why ❔

To allow running regression tests using cargo test only without putting anything to the PATH before.

Checklist

• [x] PR title corresponds to the body of PR (we generate changelog entries from PRs).
• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] Code has been formatted via cargo fmt and checked with cargo clippy.