mock: enable the mock package only in unit tests (#838)

[ti-srebot]

cherry-pick #838 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In br repo:
git pr https://github.com/pingcap/br/pull/842

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/br.git pr/842:release-4.0-969b4204aefd

---

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

[kennytm]

Seems not necessary yet since #788 is still not merged. WDYT @overvenus should this PR be directly combined into #788.

[overvenus]

Seems not necessary yet since #788 is still not merged. WDYT @overvenus should this PR be directly combined into #788.

Yes, could you pick the commit to #788 @kennytm ?