Collapse per-variant test clusters in admin-cli, libmlx, and ib-fabric

[chet]

Three more already-migrated crates carry one-function-per-variant clusters and a few tests that assert nothing. Collapsing the clusters into tables makes each decision matrix readable at a glance; the no-assertion and stdlib tests can go. Test-only; no behavior change.

Scope

ib-fabric

• Collapse the 12 should_track_port_as_down_* functions into one table (lib.rs); fold the three ib_port_down_alert_* tests into a projection table.

admin-cli

• Collapse the six dpu_mode per-variant parse tests into one scenarios! table (expected_machines/tests.rs).
• Rewrite health_override_templates_value_enum (machine/tests.rs) so it yields the variant rather than mapping a string back to itself, and so a newly-added variant can't slip silently past the _ => unreachable!() arm.
• Merge boot_override parse_get / parse_clear; drop the no-assertion routing tests (firmware / dpu parse_*) and the two assert_dispatch::<T>() compile-checks (cfg/dispatch.rs).

libmlx

• Collapse the four near-duplicate traits_tests.rs function pairs.
• Delete the five exec_options_tests.rs::advanced_tests::test_sample*_config functions (they set builder fields then re-assert them).
• Remove the construction-then-drop "creation" / "default" tests and the str::contains / Command-debug stdlib tests (tests/lockdown/, tests/runner/).

Acceptance

• Coverage preserved or improved; the value_enum rewrite fails when a variant is added without a row.
• cargo test green for the three crates; cargo clippy --all-features clean.

Size: S-M

Part of #2692.