tests: Use swaks instead of nc for sending mail (#3732)

See associated `CHANGELOG.md` entry for details.

---------

Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

test/tests/parallel/set1/spam_virus/disabled_clamav_spamassassin.bats

@@ -12,12 +12,14 @@ function setup_file() {
     --env ENABLE_CLAMAV=0
     --env ENABLE_SPAMASSASSIN=0
     --env AMAVIS_LOGLEVEL=2
+    --env PERMIT_DOCKER=container
   )
 
   _common_container_setup 'CUSTOM_SETUP_ARGUMENTS'
   _wait_for_smtp_port_in_container
 
-  _send_email 'email-templates/existing-user1'
+  _send_email --data 'existing/user1'
+  assert_success
   _wait_for_empty_mail_queue_in_container
 }