Swap bmp test images for png

Functionally it doesn't matter what kind of images we store for the
tests, as _LOADIMAGE() will open them exactly the same. png however has
the advantages of being substantially smaller, and also viewable
directly on GitHub in PRs, making them easier to review.

tests/compile_tests/paint/tile_border_black_bg.bas

@@ -13,7 +13,7 @@ _Dest test1&
 Circle (64, 25), 25, 7
 Paint (64, 25), TILE$, 7
 
-AssertImage test1&, "tile_border_black_bg.bmp"
+AssertImage test1&, "tile_border_black_bg.png"
 System
 
 '$include:'../utilities/imageassert.bm'

tests/compile_tests/paint/tile_border_black_nocolor.bas

@@ -17,7 +17,7 @@ Line (0, 0)-(127, 49), 7, BF
 Circle (64, 25), 25, 0
 Paint (64, 25), TILE$, 7
 
-AssertImage test1&, "tile_border_black_nocolor.bmp"
+AssertImage test1&, "tile_border_black_nocolor.png"
 System
 
 '$include:'../utilities/imageassert.bm'

tests/compile_tests/paint/tile_border_white_bg.bas

@@ -22,7 +22,7 @@ Pset (64, 25), 15 ' Set the starting location a third color
 
 Paint (64, 25), TILE$, 0
 
-AssertImage test1&, "tile_border_white_bg.bmp"
+AssertImage test1&, "tile_border_white_bg.png"
 System
 
 '$include:'../utilities/imageassert.bm'

tests/compile_tests/paint/tile_border_white_nocolor.bas

@@ -17,7 +17,7 @@ Line (0, 0)-(127, 49), 7, BF
 Circle (64, 25), 25, 0
 Paint (64, 25), TILE$, 7
 
-AssertImage test1&, "tile_border_white_nocolor.bmp"
+AssertImage test1&, "tile_border_white_nocolor.png"
 System
 
 '$include:'../utilities/imageassert.bm'

tests/compile_tests/paint/tile_noborder_black_bg.bas

@@ -13,7 +13,7 @@ _Dest test1&
 Circle (64, 25), 25, 7
 Paint (64, 25), TILE$
 
-AssertImage test1&, "tile_noborder_black_bg.bmp"
+AssertImage test1&, "tile_noborder_black_bg.png"
 System
 
 '$include:'../utilities/imageassert.bm'

tests/compile_tests/paint/tile_noborder_white_bg.bas

@@ -16,7 +16,7 @@ Line (0, 0)-(127, 49), 7, BF
 Circle (64, 25), 25, 0
 Paint (64, 25), TILE$
 
-AssertImage test1&, "tile_noborder_white_bg.bmp"
+AssertImage test1&, "tile_noborder_white_bg.png"
 System
 
 '$include:'../utilities/imageassert.bm'