Release over release fix by bradcray

@bradcray made these changes the last time he ran `testReleasesPerformance`. The specific issue is: "recent changes to start_test and sub_test had broken release-over-release testing because they referred directly to scripts via CHPL_HOME and therefore were trying to refer to things like chpl_platform.py which did not exist in the old releases."

@thomasvandoren reviewed, confirmed `make check` still worked, and ran a full test pass.