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.