tests: add test case to cover varchar default value (#2799)

[ti-chi-bot]

This is an automated cherry-pick of #2799

What problem does this PR solve?

Add test case to cover #2758

What is changed and how it works?

The case works as follows

• create a table, executing DML of this table in background
• alter table add column v1 varchar(20) default "xxx" not null
• Some of the DMLs could have no field of v1, however when we decode these rows, we should use the new table schema (which has the column of v1, this scenario doesn't break the constraint of online DDL). Since the column doesn't exist, the tidb library will fill in a default value, which is a string type here. This is why we can get either []byte or a string of a column.Value.

Check List

Tests

• Unit test
• Integration test

Release note

None

[ti-chi-bot]

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[amyangfei]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 7829bb6