increase coverage of color4 tests

[ax-sc]

I added more cases to test the changes introduced from Kozea/tinycss2#48 and to support #10, covering:

• hwb() function, for specifying sRGB colors in the HWB notation.
• rgb() and rgba() functions now accept <number> rather than <integer>.
• rgb(), rgba(), hsl(), and hsla() have all gained a new syntax consisting of space-separated arguments and an optional slash-separated opacity. All the color functions use this syntax form now, in keeping with CSS’s functional-notation design principles.
• All uses of <alpha-value> now accept <percentage> as well as <number>.

[liZe]

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