Run `ConcurrentTransactionTest` if `supports_transaction_isolation?`

returns true

Not only postgresql or mysql2 adapter, Oracle enhanced adapter
whose default isolation level is read commited, passes these two test cases.

`ConcurrentTransactionTest#test_transaction_per_thread`
`ConcurrentTransactionTest#test_transaction_isolation__read_committed`

```ruby
$ ARCONN=oracle bin/test test/cases/transactions_test.rb:961 -v
Using oracle
Run options: -v --seed 18865

ConcurrentTransactionTest#test_transaction_per_thread = 0.98 s = .

Finished in 1.061036s, 0.9425 runs/s, 5.6549 assertions/s.

1 runs, 6 assertions, 0 failures, 0 errors, 0 skips
```

```ruby
$ ARCONN=oracle bin/test test/cases/transactions_test.rb:979 -v
Using oracle
Run options: -v --seed 13341

ConcurrentTransactionTest#test_transaction_isolation__read_committed = 1.85 s = .

Finished in 1.928637s, 0.5185 runs/s, 10.3700 assertions/s.

1 runs, 20 assertions, 0 failures, 0 errors, 0 skips
$
```

Also, regardless it is a file based or memory based these tests could fail with SQLite3Adapter.
(Extra CR added to make lines shorter)

```ruby
$ ARCONN=sqlite3 bin/test test/cases/transactions_test.rb:961 -v
Using sqlite3
Run options: -v --seed 18815

ConcurrentTransactionTest#test_transaction_per_thread = /home/yahonda/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/sqlite3-1.3.13/lib/sqlite3/statement.rb:108:in `step':
SQLite3::BusyException: database is locked: UPDATE "topics" SET "approved" = ?, "updated_at" = ? WHERE "topics"."id" = ? (ActiveRecord::StatementInvalid)
```

```ruby
$ ARCONN=sqlite3 bin/test test/cases/transactions_test.rb:979 -v
Using sqlite3
Run options: -v --seed 25520

ConcurrentTransactionTest#test_transaction_isolation__read_committed = 0.12 s = E
/home/yahonda/.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/sqlite3-1.3.13/lib/sqlite3/statement.rb:108:in `step':
SQLite3::BusyException: database is locked: UPDATE "developers" SET "salary" = ?, "updated_at" = ?, "updated_on" = ? WHERE "developers"."id" = ? (ActiveRecord::StatementInvalid)
```

1 files changed, 13 insertions, 15 deletions

diff --git a/activerecord/test/cases/transactions_test.rb b/activerecord/test/cases/transactions_test.rb
index 91032945a6..5c8ae4d3cb 100644
--- a/activerecord/test/cases/transactions_test.rb
+++ b/activerecord/test/cases/transactions_test.rb
@@ -954,27 +954,25 @@ class TransactionsWithTransactionalFixturesTest < ActiveRecord::TestCase
   end
 end if Topic.connection.supports_savepoints?
 
-if current_adapter?(:PostgreSQLAdapter, :Mysql2Adapter)
+if ActiveRecord::Base.connection.supports_transaction_isolation?
   class ConcurrentTransactionTest < TransactionTest
     # This will cause transactions to overlap and fail unless they are performed on
     # separate database connections.
-    unless in_memory_db?
-      def test_transaction_per_thread
-        threads = 3.times.map do
-          Thread.new do
-            Topic.transaction do
-              topic = Topic.find(1)
-              topic.approved = !topic.approved?
-              assert topic.save!
-              topic.approved = !topic.approved?
-              assert topic.save!
-            end
-            Topic.connection.close
+    def test_transaction_per_thread
+      threads = 3.times.map do
+        Thread.new do
+          Topic.transaction do
+            topic = Topic.find(1)
+            topic.approved = !topic.approved?
+            assert topic.save!
+            topic.approved = !topic.approved?
+            assert topic.save!
           end
+          Topic.connection.close
         end
-
-        threads.each(&:join)
       end
+
+      threads.each(&:join)
     end
 
     # Test for dirty reads among simultaneous transactions.