mock: enable the mock package only in unit tests

[kennytm]

What problem does this PR solve?

Close #837. Allows TiDB to depend on BR without pulling in Lightning's dependencies yet.

What is changed and how it works?

Make the mock package (currently depending on backend package which in turn pulls in pebble and the whole tree) require the build tag br_test. The tag is only active in unit tests.

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
  • The mock package is no longer allowed outside of unit tests.

Side effects

Related changes

• Need to cherry-pick to the release branch

Release Note

• No release note

[overvenus]

LGTM

[glorv]

Do we need to update the project document since after this change, run go test command must always add the -tags br-test parameter.

[kennytm]

@glorv I thought we only support make test?

[glorv]

It's convenient to run go test under a single or run command like go test gihub.com/pingcap/br/pkg/lightning.backend to avoid run all the tests during development.

Developers (like me?) for community may came across this issue if they forget to add the -tags br-test when run go test manually.

[AndreMouche]

/merge

[ti-srebot]

@AndreMouche Oops! auto merge is restricted to Committers of the SIG.See the corresponding SIG page for more information. Related SIG: migrate(slack).

[AndreMouche]

It's convenient to run go test under a single or run command like go test gihub.com/pingcap/br/pkg/lightning.backend to avoid run all the tests during development.

Developers (like me?) for community may came across this issue if they forget to add the -tags br-test when run go test manually.

I would like to merge this PR, could we open another PR to fix it?

[AndreMouche]

/run-all-tests

[kennytm]

@AndreMouche well we need one more LGTM anyway 🙃

PTAL @glorv the latest commit

[glorv]

/lgtm

[glorv]

/merge

[ti-srebot]

/run-all-tests

[ti-srebot]

cherry pick to release-4.0 in PR #842

[AndreMouche]

It can not fix #837, sad. TiDB still can not work with the br's new master

[kennytm]

@AndreMouche have you reverted the protobuf upgrade in https://github.com/pingcap/tidb/pull/23183/files#diff-33ef32bf6c23acb95f5902d7097b7a1d5128ca061167ec0716715b0b9eeaa5f6L21 yet