test: integration test "br_history" is not stable

[glorv]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

See: https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/br_ghpr_unit_and_integration_test/detail/br_ghpr_unit_and_integration_test/3224/pipeline/88

I think the root cause is that the test script need some drop table ddl be finished within 10s. This is not very stable, maybe we should calculate the time delta instead.

2. What did you expect to see?

3. What did you see instead?

4. What version of BR and TiDB/TiKV/PD are you using?

[3pointer]

https://internal.pingcap.net/idc-jenkins/job/br_ghpr_unit_and_integration_test/6147/display/redirect