Checking for the presence of a `ValidationProvider` resource is
unfortunately not enough to trigger the validation auto-configuration.

If `hibernate-validator` is added on the classpath without a `javax.el`
implementation, the former will blow up on startup. So far this was a
pilot error so we didn't have to care about it. Now that we have an
auto-configuration that may lead to this error, we need to be extra
careful and check that scenario ourselves.

This commit adds an extra condition that runs as late as possible and
attempt to actually initialize a `Validator`. If that fails for whatever
reason, the auto-configuration will now back off.

Several additional tests have been added to exercise this scenario.

Closes gh-6228