restart tests: do not hard-code 5 digits

[ax3l]

WarpX simulations can run multi-million steps and beyond.

The hard-coded 5 digits for restart_file need to be replaced with an option or even better, a search & regex:

regression_testing/regtest.py

Line 724 in 4b290c5

restart_file = "%s_chk%5.5d" % (test.name, test.restartFileNum)

X-ref: BLAST-WarpX/warpx#2385