test: expand test suite with security unit tests and GDB scripted checks

test: expand test suite with security unit tests and GDB scripted checks

New test_security.c (19 tests):
- user_range_ok: 11 tests covering NULL, zero-len, valid user addr,
  kernel addr rejection, boundary spanning, overflow, max user range
- bitmap: 4 tests for set/unset/cross-byte/all-bits operations
- eflags sanitization: 4 tests verifying IOPL clearing and IF forcing

New gdb_checks.py:
- Automated GDB script for QEMU+GDB integrity checks
- Verifies heap magic, PMM bitmap frame 0, max_frames, VGA mapping
- Usage: make test-gdb

Updated Makefile:
- test-host now runs both test_utils (28) and test_security (19) = 47 tests
- Added test-gdb target for GDB scripted checks
- Total: 47 host unit tests + 19 smoke checks = 66 automated tests