pkg/lightning:move mock file for lightning into lightning package

[AndreMouche]

Signed-off-by: shirly AndreMouche@126.com

What problem does this PR solve?

move the mock files for lightning into the lightning package, since these files are only used in tests for lightning.
Meanwhile after this PR merged, tidb needn't import github.com/cockroachdb/pebble indirectly.

related PR:#786

Check List

Tests

• Unit test
• Integration test

Release Note

• No release note

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[overvenus]

Please do not split mock package.

[AndreMouche]

why? I think it is only used for lighting, so we could and should split it. If we should merge them together, so why we need to have an independent package for lighting?

Please do not split mock package.

why? I think it is only used for lighting, so we could and should split it. If we should merge them together, so why we need to have an independent package for lighting?

[kennytm]

This PR is caused by pingcap/tidb#23183.

The issue is like this:

1. After merging Lightning into BR, BR as a module now depends on Pebble.
2. Pebble depends on github.com/cockroachdb/errors
3. github.com/cockroachdb/errors depends on Protobuf 1.4.2
4. But TiDB currently requires Protobuf at =1.3.4 due to some maintenance issue or something.

Current solution in 23183 is to do replace protobuf => protobuf 1.3.4. Now trying an alternate solution of making the mocks test-only.