add test case for backslash letter commands

Using `expect` script because `\letter` aliases are
implemented only in interactive client mode.

tests/queries/0_stateless/02105_backslash_letter_commands.expect

@@ -0,0 +1,47 @@
+#!/usr/bin/expect -f
+# Tags: no-fasttest
+
+log_user 0
+set timeout 02
+match_max 100000
+# A default timeout action is to do nothing, change it to fail
+expect_after {
+    timeout {
+        exit 1
+    }
+}
+
+set basedir [file dirname $argv0]
+spawn bash -c "source $basedir/../shell_config.sh ; \$CLICKHOUSE_CLIENT_BINARY \$CLICKHOUSE_CLIENT_OPT --disable_suggestion"
+expect ":) "
+
+# Send a command
+send -- "\\ld;\r"
+expect "Syntax error: *"
+expect ":) "
+
+send -- "\\c;\r"
+expect "Syntax error: *"
+expect ":) "
+
+send -- "  \\l ;  \\d;  \r"
+expect "Syntax error (Multi-statements are not allowed): *"
+expect ":) "
+
+send -- "  \\l  ;\r"
+expect "SHOW DATABASES"
+expect "system"
+expect ":) "
+
+send -- "\\c system;\r"
+#expect "USE system"
+expect ":) "
+
+send -- "  \\d like 'one'\\G\r"
+expect "SHOW TABLES"
+expect "Row 1:"
+expect "name: one"
+expect ":) "
+
+send -- "\4"
+expect eof