bug(MatNavListItemHarness): getText() needs to exclude icons

`MatNavListItemHarness` function `getText()` is including text for icons inside the `mat-list-item`:

```
<a mat-list-item>
  <mat-icon matListIcon>help</mat-icon>
  <p>Help</p>
</a>
```

Using `getText()` for the above case currently outputs `'helpHelp'`. There should be some mechanism of getting the text without the icon.