tests/bsim/run_parallel: Create folder for results xml if missing

Create the folder for the results if it does not exist.
Fixes these warnings in CI:
```
touch: cannot touch '/__w/zephyr/zephyr/./bsim_bt/
53_bsim_results.xml': No such file or directory
realpath: /__w/zephyr/zephyr/./bsim_bt/
53_bsim_results.xml: No such file or directory
```

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>