tests(ticdc): set up the sync diff output directory correctly (#3725)

[ti-chi-bot]

This is an automated cherry-pick of #3725

What problem does this PR solve?

Use the same directory as the log.

What is changed and how it works?

set up the sync diff output directory correctly. Move it into /tmp/tidb_cdc_test/.

Check List

Tests

• Integration test

Code changes

None

Side effects

None

Related changes

• Need to cherry-pick to the release branch

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-5.1@800276f). Click here to learn what that means.
The diff coverage is n/a.

@@               Coverage Diff                @@
##             release-5.1      #3744   +/-   ##
================================================
  Coverage               ?   64.5047%           
================================================
  Files                  ?        160           
  Lines                  ?      16698           
  Branches               ?          0           
================================================
  Hits                   ?      10771           
  Misses                 ?       4983           
  Partials               ?        944           

[Rustin170506]

/merge

[ti-chi-bot]

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

Commit hash: b401d8b