Don't create a window when testing

We do this by creating the notion of a "headless" mode, and skip window
creation in SDL.

src/vm_screen.c

@@ -74,6 +74,7 @@ vm_screen_set_logical_coords(vm_screen *screen, int xcoords, int ycoords)
 int
 vm_screen_add_window(vm_screen *screen, int width, int height)
 {
+#ifndef HEADLESS
     SDL_CreateWindowAndRenderer(
         width, height, 0, &screen->window, &screen->render);
 
@@ -81,6 +82,7 @@ vm_screen_add_window(vm_screen *screen, int width, int height)
         log_critical("Could not create window: %s", SDL_GetError());
         return ERR_GFXINIT;
     }
+#endif
 
     // We plan to draw onto a surface that is xcoords x ycoords in area,
     // regardless of the actual size of the window.
@@ -150,8 +152,10 @@ vm_screen_set_color(vm_screen *screen,
                     uint8_t blue,
                     uint8_t alpha)
 {
+    if (screen->render) {
         SDL_SetRenderDrawColor(screen->render, red, green, blue, alpha);
     }
+}
 
 /*
  * Draw a rectangle on the screen at a given x/y position, with a given

tests/CMakeLists.txt

@@ -4,6 +4,8 @@ project(erc-test)
 
 set(CMAKE_BUILD_TYPE Debug)
 
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -DHEADLESS")
+
 if(DEFINED ENV{STATIC_ANALYSIS})
 	set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --analyze")
 endif()