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_wrong_command.cmy

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