elf: Add a DTV setup test [BZ #27136]

The test dlopens a large number of modules with TLS, they are reused
from an existing test.

The test relies on the reuse of slotinfo entries after dlclose, without
bug 27135 fixed this needs a failing dlopen. With a slotinfo list that
has non-monotone increasing generation counters, bug 27136 can trigger.

--
v2:
- moved from nptl/ to elf/
- dlopen a different set of existing modules.
- code style fixes (!= NULL etc).
- moved after bug fix patch