Remove compiler warning for `vol-tests/driver/h5vl_test_driver.cxx`

```
-- The C compiler identification is GNU 11.3.0
...
/home/runner/work/vol-daos/vol-daos/test/vol/driver/h5vl_test_driver.cxx:484:23: warning: ignoring return value of ‘int system(const char*)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
407
484 | system(command.c_str()); \
408
| ~~~~~~^~~~~~~~~~~~~~~~~
```

Open vol-tests/driver/h5vl_test_driver.cxx around line 484 and inspect the system(command.c_str()) call that triggers the GNU 11.3.0 warning. Run the relevant test build or CI command, then confirm the compiler no longer reports the unused-result warning.

Written by the indexing model from the issue text.