floattidf, floatuntidf: ignore differences smaller than 2 in test
floattidf, floatuntidf: ignore differences smaller than 2 in test

Its possible that the generated f64 is different from the expected one
by one bit. This is legal when both values are equally close to the i128/u128.

japaric/rustc-builtins,japaric/rustc-builtins