Fix test_sdl2_canvas_palette

Changing a color palette causes all the previous rect's using that
color to be changed to the new color. For example, in this test,
changing the palette color from green to yellow caused all the rects
that were drawn in green to change to yellow.
Add test_sdl2_key

This test doesn't check for alt, ctl or shift unlike
test_sdl_key because those keycodes(1250, 1248, 1249)
don't seem to work anymore. I've manually verified that
the keys do work.