tests: Add scan_code_imports benchmark

```console
$ ./tests/bench/scan_code
scan_code_imports python2.7  100 loops, best of 3: 3.19 msec per loop
scan_code_imports python3.9  500 loops, best of 5: 685 usec per loop
scan_code_imports python3.10  500 loops, best of 5: 727 usec per loop
scan_code_imports python3.11  500 loops, best of 5: 601 usec per loop
scan_code_imports python3.12  500 loops, best of 5: 609 usec per loop
scan_code_imports python3.13  500 loops, best of 5: 586 usec per loop
```

Author: moreati