add test case to cover issue from #4103 (#4292)

Landing this without a changeset as it's simply a test to cover a previously accepted fix.

packages/slate/test/interfaces/Editor/positions/all/inline-normalized.tsx

@@ -0,0 +1,29 @@
+/** @jsx jsx */
+import { Editor } from 'slate'
+import { jsx } from '../../../..'
+
+export const input = (
+  <editor>
+    <block>
+      <text />
+      <inline>o</inline>
+      <text />
+    </block>
+  </editor>
+)
+
+export const test = editor => {
+  return Array.from(
+    Editor.positions(editor, {
+      at: Editor.range(editor, []),
+      unit: 'character',
+    })
+  )
+}
+
+// this is the output but it's incorrect.
+// there should be two positions, before the character and after the character
+export const output = [
+  { path: [0, 0], offset: 0 },
+  { path: [0, 1, 0], offset: 1 },
+]