....and toggle it off during tests

data/mods/TEST_DATA/game_balance.json

@@ -0,0 +1,9 @@
+[
+  {
+    "type": "EXTERNAL_OPTION",
+    "name": "OVERMAP_GEOGRAPHY_CHANGE",
+    "info": "Controls whether or not density of forests and urban areas will be modified with distance from the 0,0 overmap. False disables this scaling completely.",
+    "stype": "bool",
+    "value": true
+  }
+]

tests/overmap_test.cpp

@@ -13,6 +13,7 @@
 #include "map.h"
 #include "mapbuffer.h"
 #include "omdata.h"
+#include "options.h"
 // #include "options_helpers.h"
 #include "output.h"
 #include "overmap.h"
@@ -264,6 +265,9 @@ TEST_CASE( "overmap_terrain_coverage", "[overmap][slow]" )
     point_abs_omt origin;
     map &main_map = get_map();
 
+    // Geography scaling is disabled when the TEST_DATA mod is loaded, this test performs very poorly otherwise.
+    REQUIRE( get_option<bool>( "OVERMAP_GEOGRAPHY_CHANGE" ) );
+
     for( const point_abs_omt &p : closest_points_first( origin, 0, 10 * OMAPX - 1 ) ) {
         // We need to avoid OMTs that overlap with the 'main' map, so we start at a
         // non-zero minimum radius and ensure that the 'main' map is inside that