MNT Fix link test to not pre-cache link owner before test starts.

With the new caching, this will fail tests that add versioned to the
class. That's because of a combination of factors:
- extension instances are stored on instances instead of accessed
  statically
- in `DataObject::get_one()`, the `getUniqueKeyComponents()` method is
  used to determine the cache key, which just _happens_ to be called on
a singleton, and we just _happen_ to have not generated a singleton of
the class until now, which means it has the versioned extension freshly
added to it.

This would be extremely unlikely to occur in real code, and I think our
response to a bug report would be "reset your cache after applying the
extension, or just don't fiddle around with extensions at runtime except
inside `_config.php`".

Author: GuySartorelli

tests/php/Controllers/LinkFieldControllerTest.php

@@ -13,6 +13,7 @@
 use SilverStripe\Versioned\Versioned;
 use SilverStripe\LinkField\Models\Link;
 use PHPUnit\Framework\Attributes\DataProvider;
+use SilverStripe\ORM\DataList;
 
 class LinkFieldControllerTest extends FunctionalTest
 {
@@ -50,6 +51,8 @@ protected function setUp(): void
         $link = $this->getFixtureLink();
         $owner = $this->getFixtureLinkOwner();
         $owner->Link = $link;
+        $owner->write();
+        DataList::reset(LinkOwner::class);
     }
 
     protected function tearDown(): void