tests: Avoid rounding error and correct source pixels

tests: Avoid rounding error and correct source pixels

In float-to-8bit.c and "float -> u8 6", change 0.3 to 0.301 since 0.3
/ 0.6 * 255 = 127.5, so the slightest platform specific rounding error
will give different results for u8 (127 or 128).

Also correct G'a and B'a values in the third pixel to match expected
values, the source values seems to have forgotten about
premultiplication.