Enhance tests for GBC

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Now that the help screen is enhanced for Game Boy Color, several existing tests in the Game Boy suite could benefit from enhancement as well.

• Gray ramp (32 actual steps instead of 12 quarter steps)
• Solid color screen (custom RGB color)
• Motion blur (change shade 0-3 to 0-31 and it'll substitute for 100 IRE)
• Shadow sprite (colorize the Gus portrait)
• Scroll test
• Vertical scroll test
• Lame boy demo (make it look like nrom-template)
• Grid (make border tiles red for better contrast)

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For Scroll test, the following changes are needed that can't be accomplished with the attribute plane alone:

• The sky behind the clouds should be blue on GBC or white on monochrome, but the clouds' interior should be white on both.
• The behind the grass should be blue on GBC or light gray , but the light gray portion of the grass should be medium green on GBC.

Neither of these crosses any scanline that currently uses color 3. Therefore, we don't need to include separate tile data for as we currently do with the Gus portrait in Shadow sprite. Instead, we can draw tiles with the same shade on mono but differently on GBC by changing rBGP in addition to rSCX in .waitandwrite in scrolltest.s. Fortunately, most of these change points reflect existing split points.

• 0-15: %00100100 (white, light gray, dark gray, white in cloud)
• 16-111: %11100100 (normal)
• 112-119: %01100100 white, light gray, dark gray, light gray in water)
• 120-143: %11100100 (normal; new split point)

Adding the new split point above 120 is expected to extend the runtime of set_hillzone_scroll_pos by 8 scanlines, leaving only 24 for input processing logic. We'll have to see whether this causes a problem for the other use of this routine in the Shadow sprite test.