testing: Fix race in tnc/tnccs-20-pdp-pt-tls scenario

testing: Fix race in tnc/tnccs-20-pdp-pt-tls scenario

aacf84d837e7 ("testing: Add expect-connection calls for all tests and
hosts") removed the expect-connection call for the non-existing aaa
connection.  However, because the credentials were loaded asynchronously
via start-script the clients might have been connecting when the secrets
were not yet loaded.  As `swanctl --load-creds` is a synchronous call
this change avoids that issue without having to add a sleep or failing
expect-connection call.