zdtm: Custom file names in binfmt_misc test

If several copies of binfmt_misc test are running, we may fail because of duplicate names.
Fix that by giving a custom name for registered /proc/sys/fs/binfmt_misc/xxx.

################## Test zdtm/static/binfmt_misc FAIL at start ##################
Test output: ================================
18:03:33.594: 4: string: :magic_file:M:9:\x73\x51\xff\x4a\xec\x29\xcd\xba\xab\xf2::/bin/interpreter:OCP
18:03:33.594: 4: FAIL: binfmt_misc.c:133: write magic_file (errno = 22 (Invalid argument))
18:03:33.594: 3: ERR: test.c:204: Test exited unexpectedly with code 1

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>