Implement POSIX test and [ builtin commands

[AlexandreYang]

What does this PR do?

Implements the POSIX test and [ builtin commands for conditional expression evaluation. These commands evaluate expressions and exit with status 0 (true) or 1 (false), supporting string comparisons, integer comparisons, file tests, and logical operators.

Motivation

The test and [ commands are fundamental POSIX shell builtins used throughout shell scripts for conditional logic. This implementation enables proper shell script compatibility and enables control flow constructs like if [ -f file ] to work correctly.

Testing

• Added comprehensive test suites covering:
  • String operators (-n, -z, =, !=, <, >)
  • Integer comparison operators (-eq, -ne, -gt, -ge, -lt, -le)
  • File test operators (-e, -f, -d, -s, -r, -w, -x, -L, -h, -nt, -ot, -ef)
  • Logical operators (!, -a, -o)
  • Parenthesized expressions for grouping
  • Bracket syntax ([ EXPR ])
  • Error cases (invalid integers, syntax errors, missing brackets)
  • GNU coreutils compatibility tests
  • Penetration testing edge cases (overflow, path traversal, nested expressions)
  • Unix-specific tests (symlinks, special files, permissions)
• Added scenario tests covering typical use cases
• All tests pass with proper exit codes and error messages

Checklist

• Tests added/updated
• Documentation updated (SHELL_COMMANDS.md)

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Bits Dev status: ✅ Done

CI Auto-fix: Disabled | Enable

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