tests/fuzzers/bn256: add PairingCheck fuzzer

[MariusVanDerWijden]

This PR removes Gnark from the Pair fuzzer. Because gnark uses a different pairing algorithm, it might produce a different result than the other implementations. However this is not an issue, since the bilinearity of a pairing means both results are correct.
In order to preserve some fuzzing of the gnark pairing, I added a fuzzer that fuzzes the pairingCheck function, which does a pairing and verifies that the result is not equal to one. This is the function we use in ethereum, the pairing algorithm is not under consensus, only the pairing check algorithm.

[holiman]

IMO let's keep it in one method, the inputs are the same (and probably the interesting paths/inputs too), so splitting it doesn't make much sense to me

[holiman]

Suggested change

	// gnark uses a different pairing algorithm than google & cloudflare, so the results might not match up
	// which is not a problem because of the bilinearity of the pairing.
	return 1
	}
	func FuzzPairingCheck(data []byte) int {
	input := bytes.NewReader(data)
	pc, pg, ps := getG1Points(input)
	if pc == nil {
	return 0
	}
	tc, tg, ts := getG2Points(input)
	if tc == nil {
	return 0
	}
	// Pair the two points and ensure they result in the same output
	clOK := cloudflare.PairingCheck([]*cloudflare.G1{pc}, []*cloudflare.G2{tc})
	gOK := google.PairingCheck([]*google.G1{pg}, []*google.G2{tg})
	if clOK != gOK {
	panic("pairing check mismatch: cloudflare/google")
	}
	cOK, err := bn254.PairingCheck([]bn254.G1Affine{*ps}, []bn254.G2Affine{*ts})
	// gnark uses a different pairing algorithm than google & cloudflare, so the results might not match up
	// which is not a problem because of the bilinearity of the pairing.
	// Pair the two points and ensure they result in the same output
	clOK := cloudflare.PairingCheck([]*cloudflare.G1{pc}, []*cloudflare.G2{tc})
	gOK := google.PairingCheck([]*google.G1{pg}, []*google.G2{tg})
	if clOK != gOK {
	panic("pairing check mismatch: cloudflare/google")
	}
	cOK, err := bn254.PairingCheck([]bn254.G1Affine{*ps}, []bn254.G2Affine{*ts})

[holiman]

Suggested change

	pc, pg, _ := getG1Points(input)
	pc, pg, ps := getG1Points(input)

[holiman]

Suggested change

	tc, tg, _ := getG2Points(input)
	tc, tg, ts := getG2Points(input)

[holiman]

Suggested change

compile_fuzzer tests/fuzzers/bn256 FuzzPairingCheck fuzzBn256PairingCheck