Check pair validity when adding tokens, and check liquidity

[shiki-tak]

Proposed changes

• I verified the validity of the pair when adding the token and checked the liquidity.
• I have also fixed some tests as a result.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
• I have read the CLA and signed by comment I have read the CLA Document and I hereby sign the CLA in first time contribute
• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

• Please leave the issue numbers or links related to this PR here.

Further comments

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