flaky test case: backup::disk_snap::test_conf_change

[tonyxuqqi]

https://ci.pingcap.net/blue/organizations/jenkins/tikv_ghpr_test/detail/tikv_ghpr_test/42399/tests

Stacktrace
thread 'backup::disk_snap::test_conf_change' panicked at tests/integrations/backup/disk_snap.rs:37:5:
header { error { message: ""[components/raftstore/src/store/peer.rs:4727]: [region 1] 2 peer has not applied to current term, applied_term 5, current_term 6"" } current_term: 6 }
stack backtrace:
0: rust_begin_unwind
at /rustc/89e2160c4ca5808657ed55392620ed1dbbce78d1/library/std/src/panicking.rs:645:5
1: core::panicking::panic_fmt
at /rustc/89e2160c4ca5808657ed55392620ed1dbbce78d1/library/core/src/panicking.rs:72:14
2: test_backup::disk_snap::assert_success
3: integrations::backup::disk_snap::test_conf_change
at /home/jenkins/tikv-src/tests/integrations/backup/disk_snap.rs:37:5
4: integrations::backup::disk_snap::test_conf_change::{{closure}}
at /home/jenkins/tikv-src/tests/integrations/backup/disk_snap.rs:27:22
5: core::ops::function::FnOnce::call_once
at /rustc/89e2160c4ca5808657ed55392620ed1dbbce78d1/library/core/src/ops/function.rs:250:5
6: test_util::runner::run_test_with_hook::{{closure}}::{{closure}}
7: core::ops::function::FnOnce::call_once{{vtable.shim}}
8: <alloc::boxed::Box<F,A> as core::ops::function::FnOnce>::call_once
at /rustc/89e2160c4ca5808657ed55392620ed1dbbce78d1/library/alloc/src/boxed.rs:2015:9
note: Some details are omitted, run with RUST_BACKTRACE=full for a verbose backtrace.
Standard Output
running 1 test
connecting channel to 127.0.0.1:33615 for store 1
connecting channel to 127.0.0.1:37519 for store 2
connecting channel to 127.0.0.1:38337 for store 3
connecting channel to 127.0.0.1:35863 for store 4
test backup::disk_snap::test_conf_change ... FAILED