refactor: part 2 of distinguish between Unique and UniqueIndex

[black-06]

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

What did this pull request do?

It's part 2 of #6386 which is the part of rollback, see #6386 (comment).

It will also fix the ci.

But note that it depends on

• refactor: distinguish between Unique and UniqueIndex mysql#123
• refactor: distinguish between Unique and UniqueIndex postgres#195
• refactor: distinguish between Unique and UniqueIndex sqlite#156
• refactor: distinguish between Unique and UniqueIndex sqlserver#106

After the above requests is merged, this pr still needs to update tests/go.mod to the corresponding commit id.

User Case Description

[saeidee]

To unblock new PRs, can we just resolve the CI tests issue in a dedicated PR and then decide/proceed on merging this new feature?

[black-06]

I have tested this mr with the above dependency, and it is OK.

Then we need to merge the driver mrs , and update test/go.mod here.

@jinzhu

If all goes well, we can solve all the troubles today 😭

[black-06]

My God, I tested it as test/docker-composite.yml, and it missed mysql 5.6

[jinzhu]

My God, I tested it as test/docker-composite.yml, and it missed mysql 5.6

This is the benefit of our CI, to catch oversights in our daily development ;)

[black-06]

What a thrilling day, now we have fixed it 😭