Set up database connection pool (#234)

Set up database connection pool

core/src/test/java/google/registry/persistence/EntityManagerFactoryProviderTest.java

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+// Copyright 2019 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.persistence;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import javax.persistence.EntityManager;
+import javax.persistence.EntityManagerFactory;
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+import org.testcontainers.containers.PostgreSQLContainer;
+
+/** Unit tests for {@link EntityManagerFactoryProvider}. */
+@RunWith(JUnit4.class)
+public class EntityManagerFactoryProviderTest {
+  @Rule public PostgreSQLContainer database = new PostgreSQLContainer();
+
+  private EntityManagerFactory emf;
+
+  @Before
+  public void init() {
+    emf =
+        EntityManagerFactoryProvider.create(
+            database.getJdbcUrl(), database.getUsername(), database.getPassword());
+  }
+
+  @After
+  public void destroy() {
+    emf.close();
+    emf = null;
+  }
+
+  @Test
+  public void testConnectToDatabase_success() {
+    EntityManager em = emf.createEntityManager();
+    assertThat(em.isOpen()).isTrue();
+    em.close();
+  }
+}