fix(test): resolve relative paths against cwd in file test operators

[chaliy]

Summary

• test/[ builtin and [[ ]] conditional now resolve relative file paths against cwd
• Previously [ -f test.txt ] always looked at /test.txt regardless of cd
• Both evaluate_unary and evaluate_binary (file ops) use resolve_file_path(cwd, arg)
• [[ ]] handler in interpreter uses same pattern

Test plan

• issue_291_file_test_after_cd_in_script — cd /project; /check.sh with [ -f test.txt ]
• issue_291_double_bracket_file_test_after_cd — cd /mydir; [[ -f data.json ]]

Closes #291