This commit adds a test slice for Spring Data R2DBC. When
`DataR2dbcTest` is added on a test, only Spring Data R2DBC repositories
and the infrastructure that they need is auto-configured.

Contrary to the JDBC counterpart, the `ConnectionFactory` is not swapped
by an embedded database.

See gh-19988
Co-authored-by: Mark Paluch <mpaluch@pivotal.io>