ethclient: fix error handling for header test (ethereum#22514)

The wantErr field was disused, and the error returned by HeaderByNumber
was not properly tested.

This simplifies the error checking using errors.Is and asserts that getting
an expected missing header returns ethereum.NotFound. Also adds a nil
check condition for header.Number before using big.Int's Sign method.

ethclient/ethclient_test.go

@@ -288,8 +288,9 @@ func testHeader(t *testing.T, chain []*types.Block, client *rpc.Client) {
 			want:  chain[1].Header(),
 		},
 		"future_block": {
-			block: big.NewInt(1000000000),
-			want:  nil,
+			block:   big.NewInt(1000000000),
+			want:    nil,
+			wantErr: ethereum.NotFound,
 		},
 	}
 	for name, tt := range tests {
@@ -299,10 +300,10 @@ func testHeader(t *testing.T, chain []*types.Block, client *rpc.Client) {
 			defer cancel()
 
 			got, err := ec.HeaderByNumber(ctx, tt.block)
-			if tt.wantErr != nil && (err == nil || err.Error() != tt.wantErr.Error()) {
+			if !errors.Is(err, tt.wantErr) {
 				t.Fatalf("HeaderByNumber(%v) error = %q, want %q", tt.block, err, tt.wantErr)
 			}
-			if got != nil && got.Number.Sign() == 0 {
+			if got != nil && got.Number != nil && got.Number.Sign() == 0 {
 				got.Number = big.NewInt(0) // hack to make DeepEqual work
 			}
 			if !reflect.DeepEqual(got, tt.want) {