Add BaseDnsWriterTest to opensource tree

This should have gone in with the implementation of BaseDnsWriter.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=232009971

javatests/google/registry/dns/writer/BaseDnsWriterTest.java

@@ -0,0 +1,58 @@
+// Copyright 2017 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.dns.writer;
+
+import static com.google.common.truth.Truth.assertThat;
+import static google.registry.testing.JUnitBackports.assertThrows;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/** Unit tests for {@link BaseDnsWriter}. */
+@RunWith(JUnit4.class)
+public class BaseDnsWriterTest {
+
+  static class StubDnsWriter extends BaseDnsWriter {
+
+    int commitCallCount = 0;
+
+    @Override
+    protected void commitUnchecked() {
+      commitCallCount++;
+    }
+
+    @Override
+    public void publishDomain(String domainName) {
+      // No op
+    }
+
+    @Override
+    public void publishHost(String hostName) {
+      // No op
+    }
+  }
+
+  @Test
+  public void test_cannotBeCalledTwice() {
+    StubDnsWriter writer = new StubDnsWriter();
+    assertThat(writer.commitCallCount).isEqualTo(0);
+    writer.commit();
+    assertThat(writer.commitCallCount).isEqualTo(1);
+    IllegalStateException thrown = assertThrows(IllegalStateException.class, writer::commit);
+    assertThat(thrown).hasMessageThat().isEqualTo("commit() has already been called");
+    assertThat(writer.commitCallCount).isEqualTo(1);
+  }
+}