test: enhance library lookup when building mntns root

Use a single awk script to parse the ldd output. Filter out other cases that
are clearly not libraries, such as static builds ("not a dynamic executable")
and linux-gate.so. Make the grep for vdso more specific into linux-vdso.so.

Tested:
- sudo test/zdtm.sh '^ns/.*'

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