doctest/Catch2 register tests with a string-literal name (TEST_CASE("...")),
which tree-sitter-cpp parses as an ERROR node, so every test function was
silently dropped. Adds a line-anchored regex fallback that appends one code
node per top-level TEST_CASE/TEST_CASE_TEMPLATE/SCENARIO plus a contains edge
from the file node, mirroring the Groovy Spock fallback.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>