Add annotation which bring support to test against spanner

[eddumelendez]

Is your feature request related to a problem? Please describe.

Make easy to test using spanner

Describe the solution you'd like

Looking into testing-code-that-uses-cloud-spanner I think the project can provide the infrastructure with @DataSpannerTest annotation. There are similar annotation for neo4j, jdbc, jpa and so on in the spring-boot project

Describe alternatives you've considered

• Add documentation related to it
• Provide an AbstractSpanner class

[meltsufin]

We are already developing support for testing with Spanner emulator.
See: #2357
Do you still think we need to add support for @DataSpannerTest?

[eddumelendez]

this is what we currently should do in order to test the repositories.

With the @SpannerTest or @DataSpannerTest we can provide the right auto-configurations via spring.factories and drop the config to detect repositories. However, the properties need to be set.

I am wondering how the amazing work #2357 can also help this approach. I am also thinking about testing the emulator not only against localhost but also include testcontainers.

[meltsufin]

Closing in favor of: GoogleCloudPlatform/spring-cloud-gcp#159