dovecot: Add tests for more error cases

This patch adds more tests for the dovecot library, in particular:
 - Protocol errors (invalid versions, etc.).
 - Invalid command (cli-specific test).
 - Connection breakups.

cmd/dovecot-auth-cli/test_auth_bad_proto.cmy

@@ -0,0 +1,37 @@
+
+# Break the handhake early.
+client unix_listen .dovecot-client
+c = ./dovecot-auth-cli .dovecot auth username password
+
+client <- VERSION	1	1
+client <~ CPID	
+
+# We are supposed to send the handshake here.
+client close
+
+c <- no: error receiving handshake: EOF
+c wait 0
+
+
+# Break before sending the final response.
+client unix_listen .dovecot-client
+c = ./dovecot-auth-cli .dovecot auth username password
+
+client -> VERSION	1	1
+client -> SPID	12345
+client -> CUID	12345
+client -> COOKIE	lovelycookie
+client -> MECH	PLAIN
+client -> MECH	LOGIN
+client -> DONE
+
+client <- VERSION	1	1
+client <~ CPID	
+
+client <- AUTH	1	PLAIN	service=smtp	secured	no-penalty	nologin	resp=dXNlcm5hbWUAdXNlcm5hbWUAcGFzc3dvcmQ=
+
+# We're supposed to send the OK/FAIL here.
+client close
+
+c <- no: error receiving response: EOF
+c wait 0