Unbreak wpilibc WritePreferencesToFile test. (#103)
Unbreak wpilibc WritePreferencesToFile test. (#103)

ntcore recently changed GlobalDeleteAll() to not delete persistent values.
This test was relying on the old behavior to provide a clean slate.
robotdotnet/allwpilib,333fred/allwpilib,PeterMitrano/allwpilib,robotdotnet/allwpilib,PeterMitrano/allwpilib,PeterMitrano/allwpilib,PeterMitrano/allwpilib,333fred/allwpilib,333fred/allwpilib,robotdotnet/allwpilib,robotdotnet/allwpilib,PeterMitrano/allwpilib,PeterMitrano/allwpilib,333fred/allwpilib,333fred/allwpilib,333fred/allwpilib,robotdotnet/allwpilib,PeterMitrano/allwpilib,robotdotnet/allwpilib