check_gdal regress fails if gdal_enabled_drivers already has drivers

[robe]

I thought I complained about this already, but can't find the ticket.

Anyway on my windows box where I've got GDAL_DRIVERS set, check_gdal always fails.

 check_gdal .. failed (diff expected obtained: /projects/postgis/tmp/2.2_pg9.4w64/test_1_diff)

On closer inspection, it seems the first test runs without setting drivers, and assumes the user is at the default state of -- no drivers and it also checks end state to assuming again user is running without drivers. This of course is not true when I launch my instances with something like

@SET GDAL_DRIVERS=something

or have in postgresql conf to have this set.

This might only be an issue in PostGIS 2.2 since we started using raster GUCS in this, and this is testing the raster gucs.