Support @AutoConfigureTestDatabase with multiple DataSources

[membersound]

Follow up on spring-projects/spring-data-examples#332

Problem: if multiple datasources are configured, it would still be nice if @AutoConfigureTestDatabase could pick any DataSource around and apply H2 embedded test-database settings application-wide.

spring.datasource.url=jdbc:mysql://localhost:3306/localdb
spring.datasource.username=root
spring.datasource.password=rootpw

bar.datasource.url=jdbc:mysql://remotehost:3306/remotedb
bar.datasource.username=root
bar.datasource.password=rootpw

Now it would be nice if we could just use the following:

@SpringBootApplication
@AutoConfigureTestDatabase(Replace.ANY)
public class MyJunitTest {

}

[wilkinsona]

Thanks for the suggestion. This is a small part of the changes that are being tracked by #15732.