core/forkid: fix issue in validation test (#26544)

This changes the test to match the comment description. Using timestampedConfig in this test case is incorrect, the comment says 'local is at Gray Glacier' and isn't aware of more forks.

core/forkid/forkid_test.go

@@ -308,7 +308,7 @@ func TestValidation(t *testing.T) {
 
 		// Local is mainnet Gray Glacier, and isn't aware of more forks. Remote announces Gray Glacier +
 		// 0xffffffff. Local needs software update, reject.
-		{&timestampedConfig, 15050000, 0, ID{Hash: checksumToBytes(checksumUpdate(0xf0afd0e3, math.MaxUint64)), Next: 0}, ErrLocalIncompatibleOrStale},
+		{params.MainnetChainConfig, 15050000, 0, ID{Hash: checksumToBytes(checksumUpdate(0xf0afd0e3, math.MaxUint64)), Next: 0}, ErrLocalIncompatibleOrStale},
 
 		// Local is mainnet Gray Glacier, and is aware of Shanghai. Remote announces Shanghai +
 		// 0xffffffff. Local needs software update, reject.