Update python test for encoding checks

We ensure we are below the modulus by just using a zero final byte for
each field element encoding.

In the test, I do not understand why changing the final (zero) byte
causes verification to succeed instead of failing. But this is why the
change is now to the first byte.

bindings/python/tests.py

@@ -7,7 +7,9 @@ BLOB_SIZE = 4096
 MAX_BLOBS_PER_BLOCK = 16
 
 blobs = [
-  random.randbytes(32 * BLOB_SIZE) for _ in range(3)
+  # use zero final bytes to easily ensure the encodings are valid
+  b''.join([b''.join([random.randbytes(31), bytes(1)]) for _ in range(BLOB_SIZE)])
+  for _ in range(3)
 ]
 
 ts = ckzg.load_trusted_setup("../../src/trusted_setup.txt")
@@ -26,8 +28,8 @@ assert ckzg.verify_aggregate_kzg_proof(blobs_bytes, kzg_commitments, proof, ts),
 
 # Verification fails at wrong value
 
-other = b'x' if not blobs_bytes.endswith(b'x') else b'y'
-other_bytes = blobs_bytes[:-1] + other
+other = b'x' if not blobs_bytes.startswith(b'x') else b'y'
+other_bytes = other + blobs_bytes[1:]
 
 assert not ckzg.verify_aggregate_kzg_proof(other_bytes, kzg_commitments, proof, ts), 'verify succeeded incorrectly'