cmd/devp2p/internal/ethtest: update test chain (#21742)

The old one was wrong in two ways: the first block in chain.rlp was the
genesis block, and the genesis difficulty was below minimum difficulty.

This also contains some other fixes to the test.

cmd/devp2p/internal/ethtest/testdata/genesis.json

@@ -1,6 +1,6 @@
 {
     "config": {
-        "chainId": 1,
+        "chainId": 19763,
         "homesteadBlock": 0,
         "eip150Block": 0,
         "eip155Block": 0,
@@ -11,16 +11,16 @@
     "nonce": "0xdeadbeefdeadbeef",
     "timestamp": "0x0",
     "extraData": "0x0000000000000000000000000000000000000000000000000000000000000000",
-    "gasLimit": "0x8000000",
-    "difficulty": "0x10",
+    "gasLimit": "0x80000000",
+    "difficulty": "0x20000",
     "mixHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
     "coinbase": "0x0000000000000000000000000000000000000000",
     "alloc": {
         "71562b71999873db5b286df957af199ec94617f7": {
-            "balance": "0xf4240"
+            "balance": "0xffffffff"
         }
     },
     "number": "0x0",
     "gasUsed": "0x0",
     "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000"
-}
+}