don't establish a new connection when testing with sqlite3_mem.

This fixes broken `rake test_sqlite3_mem` suite for Active Record.
The problem is that that the old database with the schema is lost
when establishing a new connection. Upon reconnting we are left
with a blank database and tests down the line start failing.

activerecord/test/cases/adapters/sqlite3/sqlite3_adapter_test.rb

@@ -34,24 +34,30 @@ def test_bad_connection
       end
 
       def test_connect_with_url
-        original_connection = ActiveRecord::Base.remove_connection
-        tf = Tempfile.open 'whatever'
-        url = "sqlite3://#{tf.path}"
-        ActiveRecord::Base.establish_connection(url)
-        assert ActiveRecord::Base.connection
-      ensure
-        tf.close
-        tf.unlink
-        ActiveRecord::Base.establish_connection(original_connection)
+        skip "can't establish new connection when using memory db" if in_memory_db?
+        begin
+          original_connection = ActiveRecord::Base.remove_connection
+          tf = Tempfile.open 'whatever'
+          url = "sqlite3://#{tf.path}"
+          ActiveRecord::Base.establish_connection(url)
+          assert ActiveRecord::Base.connection
+        ensure
+          tf.close
+          tf.unlink
+          ActiveRecord::Base.establish_connection(original_connection)
+        end
       end
 
       def test_connect_memory_with_url
-        original_connection = ActiveRecord::Base.remove_connection
-        url = "sqlite3:///:memory:"
-        ActiveRecord::Base.establish_connection(url)
-        assert ActiveRecord::Base.connection
-      ensure
-        ActiveRecord::Base.establish_connection(original_connection)
+        skip "can't establish new connection when using memory db" if in_memory_db?
+        begin
+          original_connection = ActiveRecord::Base.remove_connection
+          url = "sqlite3:///:memory:"
+          ActiveRecord::Base.establish_connection(url)
+          assert ActiveRecord::Base.connection
+        ensure
+          ActiveRecord::Base.establish_connection(original_connection)
+        end
       end
 
       def test_valid_column