Add a test.

tests/php/Core/Validation/MemberValidatorTest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace SilverStripe\Core\Tests\Validation;
+
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\Security\Member;
+use SilverStripe\Security\Member_Validator;
+
+class MemberValidatorTest extends SapphireTest
+{
+    protected $usesDatabase = false;
+
+    public function testMemberValidator(): void
+    {
+        $member = new Member();
+        $compositeValidator = $member->getCMSCompositeValidator();
+
+        $memberValidators = $compositeValidator->getValidatorsByType(Member_Validator::class);
+        $this->assertCount(1, $memberValidators, 'We expect exactly one member validator');
+    }
+}