itest: New macro, support for tmt and debian autopkgtest, and more

[cgwalters]

A lot of stuff for itest, which is our Rust-based test framework for integration testing.

Fork-exec output capture: when not running under nextest or tmt, the harness re-executes itself per test to capture stdout/stderr, matching cargo test default behavior. ITEST_NOCAPTURE=1 disables for debugging.

Test metadata generation: --emit-tmt and --emit-autopkgtest flags generate FMF and DEP-8 metadata from registered tests, enabling tmt and autopkgtest to discover and run individual tests. Verified end-to-end with tmt.

TestMeta: per-test metadata struct (timeout, needs_root, isolation, tags, summary, needs_internet, flaky) that maps to tmt and autopkgtest fields. Stored in distributed slices alongside test functions.

Proc macro (#[itest::test_attr]): attribute macro that replaces the declarative macros for cleaner ergonomics. Preserves doc comments, handles error type conversion (anyhow/eyre/io::Error), supports async fn via automatic tokio runtime creation, and accepts all metadata as attribute arguments.

VM instance type: privileged_test! and booted_test! accept itype for VM sizing, threaded through require_root() to bcvk --itype.

The Justfile now prefers nextest with cargo test fallback, and fixes a pre-existing bug where set -e skipped cleanup on test failure.

Assisted-by: OpenCode (Claude Opus 4)

[gemini-code-assist]

Code Review

This pull request introduces a new itest-macros crate to provide attribute macros for test registration, enhances the itest harness with output capture via fork-exec, and adds support for emitting test metadata in tmt (FMF) and autopkgtest (DEP-8) formats. My feedback highlights a potential naming collision in the generated static items and notes that the current output capture implementation may lose temporal ordering between stdout and stderr, though it remains acceptable for general use.