chore: make a couple of test classes public

Check if making a couple of test classes public fixes the weird native image
build error.

Author: olavloite

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/ITAbstractSpannerTest.java

@@ -65,7 +65,7 @@ public GenericConnection getConnection() {
     }
   }
 
-  protected interface ITConnection extends Connection {}
+  public interface ITConnection extends Connection {}
 
   private ITConnection createITConnection(ConnectionOptions options) {
     return new ITConnectionImpl(options);

google-cloud-spanner/src/test/java/com/google/cloud/spanner/connection/ITConnectionImpl.java

@@ -18,7 +18,7 @@
 import com.google.cloud.spanner.connection.ITAbstractSpannerTest.ITConnection;
 
 /** Implementation of {@link ITConnection} for Spanner generic (not JDBC) connections. */
-class ITConnectionImpl extends ConnectionImpl implements ITConnection {
+public class ITConnectionImpl extends ConnectionImpl implements ITConnection {
   ITConnectionImpl(ConnectionOptions options) {
     super(options);
   }