Add embeded H2 for @DataR2dbcTest

[hantsy]

Like the H2 support for @DataJpaTest , configure an embedded H2 for @DataR2dbcTest to replace the runtime driver if r2dbc-h2 is existed in the dependencies.

        <dependency>
            <groupId>org.springframework.boot.experimental</groupId>
            <artifactId>spring-boot-test-autoconfigure-r2dbc</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>io.r2dbc</groupId>
            <artifactId>r2dbc-h2</artifactId>
            <scope>test</scope>
        </dependency>

Currently it ignore h2 at all.

[mp911de]

That’s currently not possible because the embedded test database API isn’t usable from R2DBC.

Spring Boot configures a DataSource bean that does not allow introspection. Likely, if the R2DBC support gets merged into Spring Boot proper, we will be able to revisit this feature in.

[snicoll]

This experimental project has run its course and the features have moved to Spring Boot 2.3 proper. This part isn't implemented though. We brainstormed on how this feature can be improved for the plain jdbc use case and I think it could be applicable to r2dbc as well.

Please watch spring-projects/spring-boot#19038, I've just added a comment.