test/on_exit: use static variables for on_exit hooks

test/on_exit: use static variables for on_exit hooks

before this change, we allocate memory chunks using malloc(), but
we never free them. and LeakSanitizer points this out

```
Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x5588bfe532de in malloc (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_on_exit+0xa52de) (BuildId: 7c7a92bf5719592938c5307214bcd9b080bd847f)
    #1 0x5588bfe911d7 in func_scope() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/on_exit.cc:33:22
    #2 0x5588bfe90804 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/on_exit.cc:64:3
    #3 0x7f23081c1d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16

Direct leak of 4 byte(s) in 1 object(s) allocated from:
    #0 0x5588bfe532de in malloc (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_on_exit+0xa52de) (BuildId: 7c7a92bf5719592938c5307214bcd9b080bd847f)
    #1 0x5588bfe91160 in func_scope() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/on_exit.cc:29:22
    #2 0x5588bfe90804 in main /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/on_exit.cc:64:3
    #3 0x7f23081c1d8f in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
```

in this change, instead of allocating the variables using `malloc()`,
we keep them in static variables, so that they can be accessed by
`OnExitManager` even if it is a static variable.
with this change, the memory leak reports for this source file go away.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>