test: add test cases for sasl external authentication (imap/pop3/smtp).

tests/data/test945

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+<testcase>
+<info>
+<keywords>
+IMAP
+SASL
+SASL AUTH EXTERNAL
+SASL-IR
+RFC4422
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<servercmd>
+AUTH EXTERNAL
+CAPA SASL-IR
+REPLY AUTHENTICATE A002 OK AUTHENTICATE completed
+</servercmd>
+<data>
+From: me@somewhere
+To: fake@nowhere
+
+body
+
+--
+  yours sincerely
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+imap
+</server>
+ <name>
+IMAP external authentication with initial response without credentials
+ </name>
+ <command>
+'imap://;AUTH=EXTERNAL@%HOSTIP:%IMAPPORT/945/;UID=1'
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol>
+A001 CAPABILITY
+A002 AUTHENTICATE EXTERNAL =
+A003 SELECT 945
+A004 FETCH 1 BODY[]
+A005 LOGOUT
+</protocol>
+</verify>
+</testcase>