tests: use the default renderer instead of forcing opengl, unless it's set in CLI params.

testing/conf.lua

@@ -7,7 +7,6 @@ function love.conf(t)
   t.window.resizable = true
   t.window.depth = true
   t.window.stencil = true
-  t.renderers = {"opengl"}
 end
 
 -- custom crash message here to catch anything that might occur with modules