Adapt test logic to PHP and SQLite II (#9442)

In a88242e, testDateSub() was modified
in order to satisfy different opinions to the question "What is now
minus one month", that has different answers for different pieces of
software on March 30th.

Today, we are January 30th, we need to do the same for testDateAdd().
Hopefully this is the last time we hear about this.

tests/Doctrine/Tests/ORM/Functional/QueryDqlFunctionTest.php

@@ -312,8 +312,18 @@ public function testDateAdd(string $unit, int $amount, int $delta = 0): void
         self::assertArrayHasKey('now', $result);
         self::assertArrayHasKey('add', $result);
 
+        $now       = new DateTimeImmutable($result['now']);
+        $inOneUnit = $now->modify(sprintf('+%d %s', $amount, $unit));
+        if (
+            $unit === 'month'
+            && $inOneUnit->format('m') === $now->modify('+2 month')->format('m')
+            && ! $this->_em->getConnection()->getDatabasePlatform() instanceof SqlitePlatform
+        ) {
+            $inOneUnit = new DateTimeImmutable('last day of next month');
+        }
+
         self::assertEqualsWithDelta(
-            (new DateTimeImmutable($result['now']))->modify(sprintf('+%d %s', $amount, $unit)),
+            $inOneUnit,
             new DateTimeImmutable($result['add']),
             $delta
         );