chore: add test-results to gitignore

The JUnit XML reports generated by test-libc.sh --ci are build
artifacts that should not be committed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nadilas

.gitignore

@@ -13,3 +13,4 @@ tests/fixtures/fs-shim-guest/target/
 test-apps/**/target/
 test-apps/**/Cargo.lock
 __pycache__/
+/test-results