test: added getSnapshotTimestamp() method

The snapshot timestamp is the timestamp at which all the reads in a
transaction ran when using repeatable read isolation. The method is
only intended to be used for internal testing purposes, so the snapshot
timestamp will be `null` if attempted to be used externally.

Author: skuruppu

google-cloud-spanner/src/main/java/com/google/cloud/spanner/CommitResponse.java

@@ -19,6 +19,7 @@
 import com.google.cloud.Timestamp;
 import com.google.common.base.Preconditions;
 import java.util.Objects;
+import javax.annotation.Nullable;
 
 /** Represents a response from a commit operation. */
 public class CommitResponse {
@@ -41,6 +42,17 @@ public Timestamp getCommitTimestamp() {
     return Timestamp.fromProto(proto.getCommitTimestamp());
   }
 
+  /** Returns a {@link Timestamp} representing the timestamp at which all reads
+   * in the transaction ran at, if the transaction ran at repeatable read
+   * isolation in internal test environments, and otherwise returns null.
+   */
+  public @Nullable Timestamp getSnapshotTimestamp() {
+    if (proto.getSnapshotTimestamp() == com.google.protobuf.Timestamp.getDefaultInstance()) {
+      return null;
+    }
+    return Timestamp.fromProto(proto.getSnapshotTimestamp());
+  }
+
   /**
    * @return true if the {@link CommitResponse} includes {@link CommitStats}
    */