The initial stress test has ended on Classic Beta, but the stress test realm will remain accessible for play until Thursday, May 23 at 6:00 pm PDT, meaning you still have enough time to play within the level 1-5 range if you were invited, because many players had difficulties logging in to the game for many hours.
UPDATE: It seems Closed beta realms are back up for testing, or were never offline to begin with (needs confirmation).
For the time of the stress test, Closed beta realms aren’t available, but should be after the stress test realm goes down. Here’s a blue post from Blizzard:
At this time, we have wrapped up our active testing period. The stress test realm will still be available for play until Thursday, May 23 at 6:00pm PDT. If you were selected for just the stress test, there will be no realm to login to at that time tomorrow.
The closed beta realms will soon be available again for those testers.
Thank you to everyone who participated in the WoW Classic stress test!
While the stress test schedule’s fluid, the next one is planned for May 29th and the level cap will be raised to 10.
Stress Test 2: Wed May 29– Thurs May 30
Stress Test 2: Wed Jun 19– Thurs Jun 20
Stress Test 3: Thurs Jul 18– Fri July 19