It is not possible to put `@Order` on nested `@Configuration` classes and
there is no ordering guarantee of them anyway. Previously, we relied on
the declaration order to check if the `test-connection` flag should apply
to an existing `JavaMailSenderImpl`. It turns out at the
`JavaMailSenderImpl` bean was not created at that time and so the
evaluation was wrongly failing.

To make that more explicit, that code is now in its own
auto-configuration with an explicit `@AutoConfigureAfter` declaration.

Since the JNDI support could be affected by the exact same issue, it has
been restored to its own package private configuration class that is
processed before the  `MailSenderAutoConfiguration` content is evaluated.

Closes gh-3478