Reorganize the current test folder structure

[adpaco]

Depends on source-based coverage reports. The reorganization will probably follow a chapter-like structure that follows the Rust reference.

[adpaco]

Not sure what to do about this.

Organizing our tests according to the Rust reference is not something we can automate easily. There would be lots of manual labor in addition to disagreements about where to put some tests. What about reorganizing another way?