roachtest/cdc: fix cdc/schemareg expected output

Partial fix for #42690.

I don't think this ever worked. The schema change backfill for the dropped
column `b` results in a record for each row that was previously inserted.

pkg/cmd/roachtest/cdc.go

@@ -453,8 +453,11 @@ func runCDCSchemaRegistry(ctx context.Context, t *test, c *cluster) {
 		`{"before":null,"after":{"foo":{"a":{"long":2},"b":{"string":"2"}}},"updated":{"string":""}}`,
 		`{"before":null,"after":{"foo":{"a":{"long":3},"b":{"string":"3"},"c":{"long":3}}},"updated":{"string":""}}`,
 		`{"before":null,"after":{"foo":{"a":{"long":4},"c":{"long":4}}},"updated":{"string":""}}`,
+		`{"before":{"foo_before":{"a":{"long":1},"b":null,"c":null}},"after":{"foo":{"a":{"long":1},"c":null}},"updated":{"string":""}}`,
 		`{"before":{"foo_before":{"a":{"long":1},"c":null}},"after":{"foo":{"a":{"long":1},"c":null}},"updated":{"string":""}}`,
+		`{"before":{"foo_before":{"a":{"long":2},"b":{"string":"2"},"c":null}},"after":{"foo":{"a":{"long":2},"c":null}},"updated":{"string":""}}`,
 		`{"before":{"foo_before":{"a":{"long":2},"c":null}},"after":{"foo":{"a":{"long":2},"c":null}},"updated":{"string":""}}`,
+		`{"before":{"foo_before":{"a":{"long":3},"b":{"string":"3"},"c":{"long":3}}},"after":{"foo":{"a":{"long":3},"c":{"long":3}}},"updated":{"string":""}}`,
 		`{"before":{"foo_before":{"a":{"long":3},"c":{"long":3}}},"after":{"foo":{"a":{"long":3},"c":{"long":3}}},"updated":{"string":""}}`,
 	}
 	if strings.Join(expected, "\n") != strings.Join(updated, "\n") {