improve _mm256_permute2f128 tests

[folkertdev]

This came up in rust-lang/rust#148402, the zeroing behavior was untested, and the old inputs had repeating patterns that made it harder to figure out how the output is based on the input.

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[RalfJung]

This makes these tests exactly the same as the Miri tests with your changes in rust-lang/rust#148402, right?

[folkertdev]

Yes, exactly.