Previously, RedisTestServer only supported Jedis and would blow up if
only Lettuce was on the classpath. This commit defensively checks which
driver is available and chose the appropriate one, defaulting to Jedis.
Closes gh-9524
Remove test-jar artifacts from Maven projects and relocate classes. The
majority of utilities now live in the `spring-boot-testsupport` module.
This update will help us to deploy artifacts using the standard Maven
deploy plugin in the future (which doesn't support the filtering of
individual artifacts).
Fixes gh-9493
