sys-libs/glibc: disable container tests, bug #712356

sys-libs/glibc: disable container tests, bug #712356

sandbox is not very compatible with filesystem unshare().
It can't check if mkdir(/proc) is a safe operation under
unshare/chroot. Let's disable container tests.

One day we can enable them if not running under sandbox.

Shoul deal with the following test failures:

    FAIL: elf/tst-dlopen-self-container
    FAIL: elf/tst-dlopen-tlsmodid-container
    FAIL: elf/tst-ifunc-fault-bindnow
    FAIL: elf/tst-ifunc-fault-lazy
    FAIL: elf/tst-ldconfig-bad-aux-cache
    FAIL: elf/tst-ldconfig-ld_so_conf-update
    FAIL: elf/tst-pldd

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/712356
Package-Manager: Portage-2.3.94, Repoman-2.3.20
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>