Add tests to the Mail subsystem for attributes that allow expressions

The mail subsystem practically allows any attribute to use expressions, however, not all attributes are being tested at the moment.

Specifically, the use of an expression on the following attributes are not being tested automatically:

• mail-session/jndi-name
• mail-session/from
• mail-session/debug
• server-definition/outbound-socket-binding-ref
• server-definition/tls
• server-definition/ssl
• server-definition/properties