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_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