Fixed the expected error message in BIP340 verification test

This test was previously failing.

The signature verification algorithm in BIP-340 does not check that the 32
public key bytes in the signature match a valid point on the curve. This check
is effectively implicit in the algorithm later, because G and P are known valid
curve points, so the point R that is calculated is also a valid point. Thus, if
r is not a valid point's X-coordinate, R.X != r.

frost/bip340_test.go

@@ -455,7 +455,6 @@ func TestVerifySignature(t *testing.T) {
 			isValid:     false,
 			expectedErr: "point publicKey is infinite",
 		},
-
 		{
 			signature:   "FFF97BD5755EEEA420453A14355235D382F6472F8568A18B2F057A14602975563CC27944640AC607CD107AE10923D9EF7A73C643E166BE5EBEAFA34B1AC553E2",
 			publicKeyX:  "DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659",
@@ -497,7 +496,7 @@ func TestVerifySignature(t *testing.T) {
 			publicKeyX:  "DFF1D77F2A671C5F36183726DB2341BE58FEAE1DA2DECED843240F7B502BA659",
 			message:     "243F6A8885A308D313198A2E03707344A4093822299F31D0082EFA98EC4E6C89",
 			isValid:     false,
-			expectedErr: "point R is infinite",
+			expectedErr: "coordinate R.x != r",
 		},
 		{
 			signature:   "FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F69E89B4C5564D00349106B8497785DD7D1D713A8AE82B32FA79D5F7FC407D39B",