selftest/cases/glibc.py: switch to using NFS over TCP

This provides a more reliable test execution when running tests that
write a large buffer/file and significantly reduces the localedata test
failures.

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>