Update test262 runner paths for vendored test262 suite
Configure default paths to use vendor/git/test262/ for:
- test_dir: vendor/git/test262/test
- harness_dir: vendor/git/test262/harness
Current lexer status on test262/test/language/expressions:
- Pass: 10028/11093 (90.4%)
- Known issues: Unicode escapes in identifiers, some template edges
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>