justfile: Add Rust tests to pre-push — heartwood

The tests should be part of the pre-push step. Introduce test-rust, as part of a new test group, and pre-push group.

 # Run pre-push checks
 [group('hooks')]
 pre-push: check-conflict-markers format-rust check-rust check-keywords check-docs check-spelling check-scripts check-typos format-nix lint-rust
 pre-push: check-conflict-markers format-rust check-rust check-keywords check-docs check-spelling check-scripts check-typos format-nix lint-rust test-rust
     @echo ""
     @echo "{{SUCCESS}}pre-push passed!{{NORMAL}}"
     @echo ""

 [group('hooks')]
 check-hooks:
     @HINT="{{HINT}}" NORMAL="{{NORMAL}}" WARN="{{WARN}}" scripts/just/check-hooks.sh "{{hook-script}}" "{{hooks}}"
 [group('test')]
 [group('pre-push')]
 test-rust:
     @echo "{{CHECK}}Cargo test...{{NORMAL}}"
     @cargo nextest run --workspace --all-features --no-fail-fast