zdtm: add new dumpable01 test to check that dumpable flag is preserved

This confirms that the fix in commit d5bb7e97 to preserve the dumpable flag
after migration is working as expected.

In this test case, the dumpable flag is expected to always be set to 1, as
test_init will use prctl to reset it to 1 after using setuid and setgid.

Tested:
- # test/zdtm.sh static/dumpable01
  Test: zdtm/live/static/dumpable01, Result: PASS
- # test/zdtm.sh ns/static/dumpable01
  Test: zdtm/live/static/dumpable01, Result: PASS
- Confirmed that the test fails after reverting commit d5bb7e97.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>