test/crimson: fix a race condition in SeastarRunner

test/crimson: fix a race condition in SeastarRunner

This patch is supposed to fix the following problem:

```
        Start 234: unittest-seastar-errorator
216/258 Test #234: unittest-seastar-errorator ................Child aborted***Exception:   0.95 sec
WARNING: debug mode. Not for benchmarking or production
WARN  2022-01-11 10:01:32,973 [shard 0] seastar - Creation of perf_event based stall detector failed, falling back to posix timer: std::system_error (error system:13, perf_event_open() failed: Permission denied)
unittest-seastar-errorator: ../src/test/crimson/seastar_runner.h:45: int SeastarRunner::init(int, char **): Assertion `begin_signaled == true' failed.
Aborting.
Backtrace:
  0xb3397a
  0x1d19430
  0x1d1905d
  0x1b72332
  0x1b9dd95
  0x1c75fe9
  0x1c76231
  0x1c7605a
  0x7f20b7aee3bf
  /lib/x86_64-linux-gnu/libc.so.6+0x4618a
  /lib/x86_64-linux-gnu/libc.so.6+0x25858
  /lib/x86_64-linux-gnu/libc.so.6+0x25728
  /lib/x86_64-linux-gnu/libc.so.6+0x36f35
  0xc8bb88
  0xc7ec71
  /lib/x86_64-linux-gnu/libc.so.6+0x270b2
  0xafc95d
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>