test(MigrateColumn): mock alter column to improve field compare

[a631807682]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

A way for improve compare field in different drivers.
We can find the following problems
postgres:

smallint -> int2
integer -> int4 
bigint -> int8
decimal ->  numeric

mysql:

bool -> tinyint

sqlserver:

value, ok = columnType.DefaultValue()
// ok alway true, means default value not null

User Case Description

[jinzhu]

Hi @a631807682

Seems tests failed, this PR not ready?

[a631807682]

@jinzhu Yes, this PR should not be merged unless the issues that occurred in all drivers are fixed. Now its purpose is to find the problem in driver MigrateColum.