tests: add test case to cover varchar default value

[amyangfei]

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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• 3AceShowHand
• asddongmen

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[amyangfei]

/run-all-tests

[amyangfei]

/run-integration-tests

[codecov-commenter]

Codecov Report

Merging #2799 (6a33515) into master (0ebf717) will increase coverage by 1.6887%.
The diff coverage is 56.8954%.

@@               Coverage Diff                @@
##             master      #2799        +/-   ##
================================================
+ Coverage   55.4760%   57.1648%   +1.6887%     
================================================
  Files           165        180        +15     
  Lines         19841      18877       -964     
================================================
- Hits          11007      10791       -216     
+ Misses         7786       7097       -689     
+ Partials       1048        989        -59     

[amyangfei]

/merge

[ti-chi-bot]

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

Commit hash: 6a33515

[amyangfei]

/run-leak-tests
/run-kafka-tests

[amyangfei]

/run-kafka-tests

[amyangfei]

/run-kafka-tests

[amyangfei]

/run-kafka-tests

[amyangfei]

/run-kafka-tests

[amyangfei]

/run-kafka-tests

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2892.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2893.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2894.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2895.