Add tests for population of `target` field on new annotations

Add missing tests for this critical property of new annotations. In the
process revise some variable names in `createAnnotation` that were
misleading.

src/annotator/guest.js

@@ -546,15 +546,16 @@ export default class Guest extends Delegator {
     annotation.document = info.metadata;
     annotation.uri = info.uri;
 
-    // `selectors` is an array of arrays: each item is an array of selectors
-    // identifying a distinct target.
     const root = this.element;
-    const selectors = await Promise.all(
+    const rangeSelectors = await Promise.all(
       ranges.map(range => this.anchoring.describe(root, range))
     );
-    annotation.target = selectors.map(selector => ({
+    annotation.target = rangeSelectors.map(selectors => ({
       source: info.uri,
-      selector,
+
+      // In the Hypothesis API the field containing the selectors is called
+      // `selector`, despite being a list.
+      selector: selectors,
     }));
 
     this.publish('beforeAnnotationCreated', [annotation]);

src/annotator/test/guest-test.js

@@ -63,6 +63,7 @@ describe('Guest', () => {
     };
     htmlAnchoring = {
       anchor: sinon.stub(),
+      describe: sinon.stub(),
     };
     rangeUtil = {
       itemsForRange: sinon.stub().returns([]),
@@ -626,7 +627,7 @@ describe('Guest', () => {
   });
 
   describe('#createAnnotation', () => {
-    it('adds metadata to the annotation object', async () => {
+    it('adds document metadata to the annotation', async () => {
       const guest = createGuest();
       const annotation = {};
 
@@ -636,6 +637,27 @@ describe('Guest', () => {
       assert.deepEqual(annotation.document, fakeDocumentMeta.metadata);
     });
 
+    it('adds selectors for selected ranges to the annotation', async () => {
+      const guest = createGuest();
+      const fakeRange = {};
+      guest.selectedRanges = [fakeRange];
+
+      const selectorA = { type: 'TextPositionSelector', start: 0, end: 10 };
+      const selectorB = { type: 'TextQuoteSelector', exact: 'foobar' };
+      htmlAnchoring.anchor.resolves({});
+      htmlAnchoring.describe.returns([selectorA, selectorB]);
+
+      const annotation = await guest.createAnnotation({});
+
+      assert.calledWith(htmlAnchoring.describe, guest.element, fakeRange);
+      assert.deepEqual(annotation.target, [
+        {
+          source: fakeDocumentMeta.uri(),
+          selector: [selectorA, selectorB],
+        },
+      ]);
+    });
+
     it('merges properties of input object into returned annotation', async () => {
       const guest = createGuest();
       let annotation = { foo: 'bar' };