[meta] add more tests

Most of these are partially addressed in https://github.com/rust-lang/docs.rs/pull/524https://github.com/rust-lang/docs.rs/pull/524, but it would be better to have unit tests.

Off the top of my head:

• proc macros should work (but only for default target because of bugs in cargo)
• all targets are built
• anything that's not only bin is built, including cdylib, rlib, etc.
• viewing source works (#570 )
• building with docker-compose from scratch (no cache) works (expensive, doesn't need to be run on every commit)
  • including on hosts other than x86_64-unknown-linux-gnu
• renamed crates work (e.g. constellation-rs -> constellation)
• 'go to latest version' works and doesn't crash (#570 )
• A default-target other than the host builds successfully and is uploaded to the database as the default target

This would make me much more comfortable fixing https://github.com/rust-lang/docs.rs/issues/544 and https://github.com/rust-lang/docs.rs/issues/459.

Start by reviewing PR 524 and the existing unit-test setup for docs.rs. Use the listed cases as the test scope, including proc macros, all targets, renamed crates, Docker Compose builds, and non-host default targets. Done means the remaining unchecked behaviors have regression tests, while the cases marked complete remain covered.