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

cmd/dovecot-auth-cli/test_exists_bad_proto.cmy

@@ -0,0 +1,32 @@
+
+# Invalid version
+userdb unix_listen .dovecot-userdb
+c = ./dovecot-auth-cli .dovecot exists username
+
+userdb -> VERSION	0
+c <~ no: error receiving version
+c wait 0
+
+
+# No SPID (send "NOSPID" instead
+userdb unix_listen .dovecot-userdb
+c = ./dovecot-auth-cli .dovecot exists username
+
+userdb -> VERSION	1	1
+userdb -> NOSPID
+c <~ no: error receiving SPID:
+c wait 0
+
+# Break before sending the final response.
+userdb unix_listen .dovecot-userdb
+c = ./dovecot-auth-cli .dovecot exists username
+
+userdb -> VERSION	1	1
+userdb -> SPID	12345
+userdb <- VERSION	1	1
+userdb <- USER	1	username	service=smtp
+
+userdb close
+
+c <- no: error receiving response: EOF
+c wait 0

cmd/dovecot-auth-cli/test_exists_error.cmy

@@ -0,0 +1,15 @@
+
+userdb unix_listen .dovecot-userdb
+
+c = ./dovecot-auth-cli .dovecot exists username
+
+userdb -> VERSION	1	1
+userdb -> SPID	12345
+
+userdb <- VERSION	1	1
+userdb <- USER	1	username	service=smtp
+
+userdb -> OTHER
+
+c <~ no: invalid response:
+c wait 0

cmd/dovecot-auth-cli/test_wrong_command.cmy

@@ -0,0 +1,4 @@
+
+c = ./dovecot-auth-cli .missingsocket something
+c <- unknown subcommand
+c wait 0