You can use SimpleNamingContextBuilder to make a jndi datasource available to your tests:
SimpleNamingContextBuilder builder = new SimpleNamingContextBuilder();
builder.bind("java:comp/env/jdbc/mydatasource", dataSource);
builder.activate();
https://fisheye.springsource.org/browse/spring-framework/spring-test/src/main/java/org/springframework/mock/jndi/SimpleNamingContextBuilder.java?hb=true
This isn’t exactly mocking the datasource, but it does make the datasource available via jndi for your tests.