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

[hyoklee]

 -- 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
     |                 ~~~~~~^~~~~~~~~~~~~~~~~