[specs] add test for nested paging in xml

Please add a test for a scenario like this

```tsp
@service
namespace Test;

/** The blob flat list segment. */
model BlobFlatListSegment {
/** The blob items. */
@pageItems
@Xml.name("Blob")
@Xml.unwrapped
blobItems: string[];
}

/** An enumeration of blobs. */
@Xml.name("EnumerationResults")
model ListBlobsFlatSegmentResponse {
/** The container name. */
@Xml.attribute
@Xml.name("ContainerName")
containerName: string;

/** The blob segment. */
@Xml.name("Blobs")
segment: BlobFlatListSegment;

/** The next marker of the blobs. */
@continuationToken
@Xml.name("NextMarker")
nextMarker?: string;
}

@list
@route("test")
op listBlobFlatSegment(
@continuationToken
@query
marker?: string;
): {
/** The list of blobs */
@body
enumerationResults: ListBlobsFlatSegmentResponse;
}
```

You can choose clearer names and test more scenarios