Followup to #2848952 - fix DatabaseLegacyTest::testDbMerge

From #2848952-61: Replace all calls to db_merge(), which is deprecated:

+++ b/core/tests/Drupal/KernelTests/Core/Database/DatabaseLegacyTest.php
@@ -172,4 +171,22 @@ public function testDbCreateTable() {
+    $result = db_merge('test_people')
+      ->key('job', 'Presenter')
+      ->fields([
+        'age' => 31,
+        'name' => 'Tiffany',
+      ])
+      ->execute();

Needs follow-up to change test to "instanceOf" assertion because we must test that this function returns declared object - but instead it tests that object works.