Fix flaky test by freezing time before write operation (#57904)

This prevents timing race conditions where time can advance between
when the session is written (which records a timestamp) and when
the garbage collection check occurs, causing intermittent test failures.

Author: sumaiazaman

tests/Session/ArraySessionHandlerTest.php

@@ -100,6 +100,7 @@ public function test_it_cleans_up_old_sessions()
 
         $this->assertSame(0, $handler->gc(300));
 
+        Carbon::setTestNow(Carbon::now());
         $handler->write('foo', 'bar');
         $this->assertSame(0, $handler->gc(300));
         $this->assertSame('bar', $handler->read('foo'));