MDEV-18352 Add a regression test for VARCHAR enlarging

Add a simplest regression test. Specifically, I want to be sure that
SYS_COLUMNS.LEN is increased.

Closes #1123

mysql-test/suite/innodb/r/instant_varchar_enlarge.result

+create table t (a varchar(100)) engine=innodb;
+select name, pos, mtype, prtype, len from information_schema.innodb_sys_columns where name='a';
+name	pos	mtype	prtype	len
+a	0	1	524303	100
+alter table t modify a varchar(110), algorithm=inplace;
+select name, pos, mtype, prtype, len from information_schema.innodb_sys_columns where name='a';
+name	pos	mtype	prtype	len
+a	0	1	524303	110
+drop table t;

mysql-test/suite/innodb/t/instant_varchar_enlarge.test

+--source include/have_innodb.inc
+
+# LEN must increase here
+create table t (a varchar(100)) engine=innodb;
+select name, pos, mtype, prtype, len from information_schema.innodb_sys_columns where name='a';
+alter table t modify a varchar(110), algorithm=inplace;
+select name, pos, mtype, prtype, len from information_schema.innodb_sys_columns where name='a';
+drop table t;