add test case

tests/script/general/parser/alter_column.sim

+system sh/stop_dnodes.sh
+
+system sh/deploy.sh -n dnode1 -i 1
+system sh/cfg.sh -n dnode1 -c walLevel -v 1
+system sh/exec.sh -n dnode1 -s start
+sleep 100
+sql connect
+
+$dbPrefix = m_alt_db
+$tbPrefix = m_alt_tb
+$mtPrefix = m_alt_mt
+$tbNum = 10
+$rowNum = 5
+$totalNum = $tbNum * $rowNum
+$ts0 = 1537146000000
+$delta = 600000
+print ========== alter.sim
+$i = 0
+$db = $dbPrefix . $i
+$mt = $mtPrefix . $i
+
+sql drop database if exists $db
+sql create database $db
+sql use $db
+##### alter table test, simeplest case
+sql create table tb (ts timestamp, c1 int, c2 binary(10), c3 nchar(10))
+sql insert into tb values (now, 1, "1", "1")
+sql alter table tb alter column length c2 20;
+if $rows != 0 then
+  return -1
+endi
+sql alter table tb alter column length c3 20;
+if $rows != 0 then
+  return -1
+endi
+
+##### ILLEGAL OPERATIONS
+
+# try dropping columns that are defined in metric
+sql_error alter table tb alter column length c1 10;
+
+
+system sh/exec.sh -n dnode1 -s stop -x SIGINT