Add timeout to avoid nested deadlock flakey test

Due to the large number of times I ran the whole suite to debugging
other flakey tests, I was able to reproduce the issue similar to rails#49841.

That issue has still happened recently, but as we've noted (can't find
the ticket now) that the Active Record tests sometimes run other
adapter's tests leading to confusion that it might be specific to
sqlite, but it's not.

I can reproduce this by running the tests with mysql2 many many times,
eventually it hangs. And when it does the last thing in `debug.log`:

```
ActiveRecord::NestedDeadlockTest::Sample Pluck (0.1ms)  SELECT `samples`.`value` FROM `samples`
 (5.3ms)  DROP TABLE IF EXISTS `samples`
 (1.1ms)  DROP TABLE IF EXISTS `samples`
 (7.1ms)  CREATE TABLE `samples` (`id` bigint NOT NULL AUTO_INCREMENT PRIMARY KEY, `value` int)
TRANSACTION (0.0ms)  BEGIN
ActiveRecord::NestedDeadlockTest::Sample Create (0.2ms)  INSERT INTO `samples` (`value`) VALUES (?)  [["value", 1]]
TRANSACTION (1.4ms)  COMMIT
TRANSACTION (0.0ms)  BEGIN
ActiveRecord::NestedDeadlockTest::Sample Create (0.1ms)  INSERT INTO `samples` (`value`) VALUES (?)  [["value", 2]]
TRANSACTION (1.6ms)  COMMIT
TRANSACTION (0.2ms)  BEGIN
TRANSACTION (0.1ms)  BEGIN
ActiveRecord::NestedDeadlockTest::Sample Load (0.3ms)  SELECT `samples`.* FROM `samples` LIMIT ?  [["LIMIT", 1]]
TRANSACTION (0.0ms)  SAVEPOINT active_record_1
ActiveRecord::NestedDeadlockTest::Sample Load (0.4ms)  SELECT `samples`.* FROM `samples` LIMIT ?  [["LIMIT", 1]]
ActiveRecord::NestedDeadlockTest::Sample Load (0.4ms)  SELECT `samples`.* FROM `samples` WHERE `samples`.`id` = ? LIMIT ? FOR UPDATE  [["id", 2], ["LIMIT", 1]]
TRANSACTION (0.1ms)  SAVEPOINT active_record_1
TRANSACTION (0.0ms)  ROLLBACK TO SAVEPOINT active_record_1
ActiveRecord::NestedDeadlockTest::Sample Load (0.3ms)  SELECT `samples`.* FROM `samples` WHERE `samples`.`id` = ? LIMIT ? FOR UPDATE  [["id", 1], ["LIMIT", 1]]
TRANSACTION (0.4ms)  ROLLBACK
```

I tried gdb, strace, lsof, but the logs were always on the same test
class when it happened. I have a coredump if anyone one the team wants
to take a look.

Author: zzak

activerecord/test/cases/adapters/abstract_mysql_adapter/nested_deadlock_test.rb

@@ -48,7 +48,7 @@ class Sample < ActiveRecord::Base
               Sample.transaction(requires_new: true) do
                 assert_current_transaction_is_savepoint_transaction
                 s1.lock!
-                barrier.wait
+                barrier.wait(30)
                 s2.update value: 1
               end
             end
@@ -60,7 +60,7 @@ class Sample < ActiveRecord::Base
               Sample.transaction(requires_new: true) do
                 assert_current_transaction_is_savepoint_transaction
                 s2.lock!
-                barrier.wait
+                barrier.wait(30)
                 s1.update value: 2
               end
             end
@@ -91,7 +91,7 @@ class Sample < ActiveRecord::Base
           Sample.transaction(requires_new: true) do
             assert_current_transaction_is_savepoint_transaction
             s1.lock!
-            barrier.wait
+            barrier.wait(30)
             s2.update value: 4
 
           rescue ActiveRecord::Deadlocked
@@ -113,7 +113,7 @@ class Sample < ActiveRecord::Base
           Sample.transaction(requires_new: true) do
             assert_current_transaction_is_savepoint_transaction
             s2.lock!
-            barrier.wait
+            barrier.wait(30)
             s1.update value: 3
           rescue ActiveRecord::Deadlocked
             deadlocks += 1
@@ -143,7 +143,7 @@ class Sample < ActiveRecord::Base
             Sample.transaction(requires_new: true) do
               assert_current_transaction_is_savepoint_transaction
               s1.lock!
-              barrier.wait
+              barrier.wait(30)
               s2.update value: 4
             end
           rescue ActiveRecord::Deadlocked
@@ -160,7 +160,7 @@ class Sample < ActiveRecord::Base
             Sample.transaction(requires_new: true) do
               assert_current_transaction_is_savepoint_transaction
               s2.lock!
-              barrier.wait
+              barrier.wait(30)
               s1.update value: 3
             end
           rescue ActiveRecord::Deadlocked