Add mail server connection check

If a `JavaMailSenderImpl` is available, check that the underlying mail
server is available on startup. Add a `spring.mail.test-connection`
property to control this behaviour.

Closes gh-3408

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -519,6 +519,7 @@ content into your application; rather pick only the properties that you need.
 	spring.mail.default-encoding=UTF-8 # encoding to use for MimeMessages
 	spring.mail.properties.*= # properties to set on the JavaMail session
 	spring.mail.jndi-name= # JNDI location of a Mail Session
+	spring.mail.test-connection= # Enable test connection. Default: false
 
 	# SPRING BATCH ({sc-spring-boot-autoconfigure}/batch/BatchProperties.{sc-ext}[BatchProperties])
 	spring.batch.job.names=job1,job2