test: cases::test_rawkv::test_region_merge is unstable

[pingyu]

Bug Report

What version of TiKV are you using?

master (1a9446f)

What operating system and CPU are you using?

CI env.

Steps to reproduce

Run CI.

What did you expect?

cases::test_rawkv::test_region_merge succeed.

What did happened?

cases::test_rawkv::test_region_merge failed.

[2022-10-10T06:25:34.453Z] --- TRY 3 STDERR:                              tests::failpoints cases::test_rawkv::test_region_merge ---
[2022-10-10T06:25:34.453Z] thread 'cases::test_rawkv::test_region_merge' panicked at 'region merge failed', tests/failpoints/cases/test_rawkv.rs:118:17
[2022-10-10T06:25:34.453Z] stack backtrace:
[2022-10-10T06:25:34.453Z]    0: std::panicking::begin_panic
[2022-10-10T06:25:34.453Z]    1: failpoints::cases::test_rawkv::TestSuite::must_merge_region_by_key
[2022-10-10T06:25:34.453Z]              at /home/jenkins/tikv-src/tests/failpoints/cases/test_rawkv.rs:118:17
[2022-10-10T06:25:34.453Z]    2: failpoints::cases::test_rawkv::test_region_merge
[2022-10-10T06:25:34.453Z]              at /home/jenkins/tikv-src/tests/failpoints/cases/test_rawkv.rs:246:9
[2022-10-10T06:25:34.453Z]    3: failpoints::cases::test_rawkv::test_region_merge::{{closure}}
[2022-10-10T06:25:34.453Z]              at /home/jenkins/tikv-src/tests/failpoints/cases/test_rawkv.rs:205:1
[2022-10-10T06:25:34.453Z]    4: core::ops::function::FnOnce::call_once
[2022-10-10T06:25:34.453Z]              at /rustc/0f4bcadb46006bc484dad85616b484f93879ca4e/library/core/src/ops/function.rs:248:5
[2022-10-10T06:25:34.453Z]    5: test_util::runner::run_test_with_hook::{{closure}}::{{closure}}
[2022-10-10T06:25:34.453Z]    6: core::ops::function::FnOnce::call_once{{vtable.shim}}
[2022-10-10T06:25:34.453Z]    7: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once
[2022-10-10T06:25:34.453Z]              at /rustc/0f4bcadb46006bc484dad85616b484f93879ca4e/library/alloc/src/boxed.rs:1935:9
[2022-10-10T06:25:34.453Z] note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Refer to:
https://ci.pingcap.net/blue/organizations/jenkins/tikv_ghpr_test/detail/tikv_ghpr_test/27741/pipeline/
https://ci.pingcap.net/blue/organizations/jenkins/tikv_ghpr_test/detail/tikv_ghpr_test/27748/pipeline/