By default, `@DataJpaTest` (and `@AutoConfigureTestDatabase`) attempt to
replace any existing `DataSource` by an embedded one. Previously, if
there is was no embedded database on the classpath, the exception message
did not provide that context in the error message.
This commit clarifies the error message to conduct
`TestDatabaseAutoConfiguration` (that is replacing the existing
`DataSource`).
Closes gh-7797
ffa6d6d6e0