tests/br/compatibility: fix failed for manifest unknown

[Leavrth]

What problem does this PR solve?

1. compatibility test failed because there has not been docker image yet for v5.1.0-alpha.

• TAG=v5.1.0-alpha PORT_SUFFIX=3 docker-compose -p v5.1.0-alpha -f compatibility/backup_cluster.yaml up -d
• Successfully built 8119b2b49235
• Successfully tagged control:nightly
• Successfully built e3e942a2f342
• Successfully tagged control:nightly
• TAG=v5.0.1 PORT_SUFFIX=1 docker-compose -p v5.0.1 -f compatibility/backup_cluster.yaml up -d
• TAG=v4.0.13 PORT_SUFFIX=2 docker-compose -p v4.0.13 -f compatibility/backup_cluster.yaml up -d
• Creating network "v510-alpha_default" with the default driver
• Pulling pd0 (pingcap/pd:v5.1.0-alpha)...
• Creating network "v4013_default" with the default driver
• Creating network "v501_default" with the default driver
• Pulling pd0 (pingcap/pd:v4.0.13)...
• Pulling pd0 (pingcap/pd:v5.0.1)...
• manifest for pingcap/pd:v5.1.0-alpha not found: manifest unknown: manifest unknown
• v4.0.13: Pulling from pingcap/pd
• v5.0.1: Pulling from pingcap/pd`

2. source compatibility/prepare_backup.sh in tests/run_compatible.sh will trigger prepare shell again, which might be into infinite call loop.

What is changed and how it works?

1. add file check for restore to skip when prepare is not finished since there is no docker image. just filter alpha tag
2. move the function getLastestTags() into file compatibility/get_last_tags.sh

Check List

Related changes

• Need to cherry-pick to the release branch

Release note

-No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• 3pointer
• glorv

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[glorv]

/merge

[ti-chi-bot]

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

Commit hash: c6764f3

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1168.

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #1169.

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/br#release-4.0 from head ti-chi-bot:cherry-pick-1161-to-release-4.0: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-1161-to-release-4.0."}],"documentation_url":"https://docs.github.com/rest/reference/pulls#create-a-pull-request"}

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/br#release-5.0 from head ti-chi-bot:cherry-pick-1161-to-release-5.0: status code 422 not one of [201], body: {"message":"Validation Failed","errors":[{"resource":"PullRequest","code":"custom","message":"A pull request already exists for ti-chi-bot:cherry-pick-1161-to-release-5.0."}],"documentation_url":"https://docs.github.com/rest/reference/pulls#create-a-pull-request"}