ebs br: enhance TC validation (#5659)

[ti-chi-bot]

This is an automated cherry-pick of #5659

What problem does this PR solve?

Closes #5658

What is changed and how does it work?

For volume backup, this PR enhances the validation against the target TiDB cluster. That is if there is no TiKV stores or TiKV replica number is 0, the backup will fail.

Code changes

• [x] Has Go code change
• [ ] Has CI related scripts change

Tests

• [x] Unit test
• [ ] E2E test
• [ ] Manual test
• [ ] No code

Side effects

• [ ] Breaking backward compatibility
• [ ] Other side effects:

Related changes

• [ ] Need to cherry-pick to the release branch
• [ ] Need to update the documentation

Release Notes

Please refer to Release Notes Language Style Guide before writing the release note.

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Once this PR has been reviewed and has the lgtm label, please assign niubell for approval. For more information see the Code Review Process.

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

Needs approval from an approver in each of these files: - **[OWNERS](https://github.com/pingcap/tidb-operator/blob/release-1.6/OWNERS)** Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Please upload report for BASE (release-1.6@27d419c). Learn more about missing BASE report.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release-1.6 #5663 +/- ## ============================================== Coverage ? 61.48% ============================================== Files ? 246 Lines ? 34585 Branches ? 0 ============================================== Hits ? 21266 Misses ? 11322 Partials ? 1997 ``` | [Flag](https://app.codecov.io/gh/pingcap/tidb-operator/pull/5663/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | Coverage Δ | | |---|---|---| | [e2e](https://app.codecov.io/gh/pingcap/tidb-operator/pull/5663/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `21.04% <33.33%> (?)` | | | [unittest](https://app.codecov.io/gh/pingcap/tidb-operator/pull/5663/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pingcap) | `61.48% <100.00%> (?)` | |

[BornChanger]

/run-pull-e2e-kind-br