Improve ui-test setup

[joshlf]

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