Add test to check default date used by `suggestFilename`

The `/* istanbul ignore next */` comment was not working as expected. Just
writing the test was the easiest solution.

src/sidebar/helpers/export-annotations.ts

@@ -32,7 +32,6 @@ export type SuggestedFilenameOptions = {
 export function suggestedFilename({
   documentMetadata,
   groupName,
-  /* istanbul ignore next - test seam */
   date = new Date(),
 }: SuggestedFilenameOptions) {
   const filenameSegments = [formatDate(date)];

src/sidebar/helpers/test/export-annotations-test.js

@@ -51,4 +51,10 @@ describe('suggestedFilename', () => {
       );
     });
   });
+
+  it('defaults to current date', () => {
+    const result = suggestedFilename({});
+    const dateStr = new Date().toISOString().slice(0, 10);
+    assert.equal(result, `${dateStr}-Hypothesis`);
+  });
 });