test/crimson/test_messenger_thrash: fix local variable out of scope in async task

test/crimson/test_messenger_thrash: fix local variable out of scope in async task

test_messenger_thrash UT shows,

```
==461141==ERROR: AddressSanitizer: stack-use-after-return on address 0xffffb0b37c20 at pc 0xaaaad7239508 bp 0xffffeb113c50 sp 0xffffeb113c48
READ of size 4 at 0xffffb0b37c20 thread T0
    #0 0xaaaad7239504 in (anonymous namespace)::SyntheticWorkload::wait_for_done()::'lambda0'()::operator()() const /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/test/crimson/test_messenger_thrash.cc:455:13
    #1 0xaaaad723a1c0 in seastar::internal::do_until_state<(anonymous namespace)::SyntheticWorkload::wait_for_done()::'lambda'(), (anonymous namespace)::SyntheticWorkload::wait_for_done()::'lambda0'()>::run_and_dispose() /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/include/seastar/core/loop.hh:303:26
    #2 0xaaaadacfb790 in seastar::reactor::run_tasks(seastar::reactor::task_queue&) /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/reactor.cc:2653:14
    #3 0xaaaadad04288 in seastar::reactor::run_some_tasks() /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/reactor.cc:3123:9
    #4 0xaaaadad07cd0 in seastar::reactor::do_run() /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/reactor.cc:3291:9
    #5 0xaaaadad05d60 in seastar::reactor::run() /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/reactor.cc:3181:16
    #6 0xaaaadaa860d8 in seastar::app_template::run_deprecated(int, char**, std::function<void ()>&&) /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/app-template.cc:276:31
    #7 0xaaaadaa83fb0 in seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&) /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/seastar/src/core/app-template.cc:167:12
    #8 0xaaaad7203d88 in main /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/test/crimson/test_messenger_thrash.cc:669:14
    #9 0xffffb32773f8 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #10 0xffffb32774c8 in __libc_start_main csu/../csu/libc-start.c:392:3
    #11 0xaaaad712546c in _start (/home/jenkins-build/build/workspace/ceph-pull-requests-arm64/build/bin/unittest-seastar-messenger-thrash+0x3b5546c) (BuildId: b0048d750e057d178816f94b3ce0459971785191)
```

Address 0xffffb0b37c20 is located in stack of thread T0 at offset 32 in frame
    #0 0xaaaad7493ed8 in ceph::buffer::v15_2_0::list::buffers_t::clear_and_dispose() /home/jenkins-build/build/workspace/ceph-pull-requests-arm64/src/include/buffer.h:596

Signed-off-by: arm7star <arm7star@qq.com>