Improve ui-test setup

With #2994 landed, we can now follow on with various improvements. #2997 already represents a significant improvement over our old, trybuild-based setup.
- Remove "large type has been written to a file" output (which we can do using ui-test's "stderr filters") since these have nondeterministic names
- Add inline annotations for errors in `.rs` files
- Clean up `[zerocopy-derive/]tests/ui.rs`, `testutil/src/lib.rs`, and `tools/ui-runner/src/main.rs`, including addressing FIXME comments
- Be more consistent about how we do or don't propagate CLI flags or `RUSTFLAGS` from the outer invocation
- Figure out how to trick rustc into believing that the terminal is a fixed, consistent width regardless of what it really is so that it always outputs consistent `.stderr` files regardless of environment
- Run `cargo build` rather than `cargo check`, move `src/doctests.rs` tests back into UI tests