Turn all tests into integration tests

[TDecking]

For now, the bare minimum regarding #440.

Later PRs will do a couple of overhauls to the test suite.

[alejandro-vaz]

a few notes for future changes:

• keep draining tests/main.rs
• remove the test_ prefix on all test names
• get rid of test-level imports, have them all top-of-the-file (they were previously needed because we had all in one file, now we link conditionally)
• either have all the tests with #[should_panic] include a reason, or make them all not have a reason, but have it consistent
• the codebase is getting uncomfortably large, we have to try to get rid of unnecessary stuff and simplify

this PR looks good

[alejandro-vaz]

also, you can assign yourself to issues now by commenting on any issue smallbot! assign

it's all on the wiki if you want to take a look