CTest follow-up: use env variable to set `warpx.abort_on_warning_threshold` default

Follow-up on #5068. We want to use an environment variable, that will be set true for our CI applications, in order to set the default value `warpx.abort_on_warning_threshold = low` in WarpX, so that this can be correctly overwritten by our test input files if need be, without incurring into issues related to pre-prending/appending runtime parameters from the command line.