vdso/zdtm: Add iterative proxification test

Test for previously fixed bugs for vdso-trampolines insertion:
- unmapping original vvar (which went unnoticed)
- leaving rt-vvar after each C/R cycle and resulting pollution

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>