Extract `LoadingSpinner`

Unmock in `ShareDialog` tests for the moment as this is a very simple
component.

src/sidebar/components/ShareDialog/LoadingSpinner.tsx

+import { Spinner } from '@hypothesis/frontend-shared';
+
+/**
+ * Render a consistently sized and positioned loading spinner for dialog or
+ * tab content
+ */
+export default function LoadingSpinner() {
+  return (
+    <div
+      className="flex flex-row items-center justify-center"
+      data-testid="loading-spinner"
+    >
+      <Spinner size="md" />
+    </div>
+  );
+}

src/sidebar/components/ShareDialog/ShareDialog.tsx

@@ -9,7 +9,6 @@ import {
   Input,
   InputGroup,
   LockIcon,
-  Spinner,
   TabList,
   Tab,
 } from '@hypothesis/frontend-shared';
@@ -24,17 +23,7 @@ import { useSidebarStore } from '../../store';
 import { copyText } from '../../util/copy-to-clipboard';
 import ShareLinks from '../ShareLinks';
 import SidebarPanel from '../SidebarPanel';
-
-function LoadingSpinner() {
-  return (
-    <div
-      className="flex flex-row items-center justify-center"
-      data-testid="loading-spinner"
-    >
-      <Spinner size="md" />
-    </div>
-  );
-}
+import LoadingSpinner from './LoadingSpinner';
 
 /**
  * Render a header to go above a Card, with contents in a TabList

src/sidebar/components/ShareDialog/test/ShareDialog-test.js

@@ -45,6 +45,8 @@ describe('ShareDialog', () => {
     };
 
     $imports.$mock(mockImportedComponents());
+    // Don't mock this very simple component
+    $imports.$restore({ './LoadingSpinner': true });
     $imports.$mock({
       '../../store': { useSidebarStore: () => fakeStore },
       '../../helpers/annotation-sharing': {