Tests now Fail w/o Plotfile Output

[ax3l]

WarpX broke from a recent change in #135
X-ref BLAST-WarpX/warpx#4881 (comment)

From @lucafedeli88

Tests are failing due to a recent change in the regtest.py script (https://github.com/AMReX-Codes/regression_testing/blame/c17887e4afabf83a56723d30899e13264439ba8b/regtest.py#L1213). The new logic checks for files matching the pattern re.match(f"{test.name}.*_(plt|chk)[0-9]+", pfile)) , and throws an error if no file is found. However, in WarpX tests, we don't always generate such files. In some cases the output consists in just reduced diagnostics, while in others it consists in OpenPMD files.

[zingale]

this has been made an option, so things should work now by default