Fix RegExp.test fancy fallback

[andrewtdiz]

Closes #2723

Summary:

• Route RegExp.prototype.test through the existing fancy_regex fallback cache when the regex crate used the never-matching placeholder.
• Preserve the existing regex crate path for ordinary patterns.
• Add focused parity coverage for positive, negative, missing, built, and dynamic lookbehind .test calls, with match/exec controls nearby.

Pre-fix reproduction:

• env RUSTC_WRAPPER= PATH="/root/.npm/_npx/8758e404b5eed2f3/node_modules/.bin:$PATH" ./run_parity_tests.sh --suite node-suite --module globals --filter regexp-test-lookbehind failed with .test lookbehind returning false while match/exec controls succeeded (test-parity/reports/parity_report_20260530_001928.json).

Validation:

• PATH="/root/.npm/_npx/8758e404b5eed2f3/node_modules/.bin:$PATH" node test-parity/node-suite/globals/regexp-test-lookbehind.ts
• env RUSTC_WRAPPER= PATH="/root/.npm/_npx/8758e404b5eed2f3/node_modules/.bin:$PATH" ./run_parity_tests.sh --suite node-suite --module globals --filter regexp-test-lookbehind (PASS; test-parity/reports/parity_report_20260530_002338.json)
• env RUSTC_WRAPPER= cargo test -p perry-runtime regex
• env RUSTC_WRAPPER= cargo check -p perry-runtime
• env RUSTC_WRAPPER= cargo fmt --all -- --check
• git diff --check
• jq empty test-parity/known_failures.json test-parity/threshold.json
• ./scripts/check_file_size.sh