ltp-testsuite: disable tirpc tests using authdes_create

In buildroot, libtirpc is patched to remove authdes_create functions.
As a result, compilation of tests that use these functions, fails.

A previous fix was implemented in commit
8f1c4c49, but simply disabled linking with
libtirpc entirely. This broke usage of ltp-testsuite on targets where the
toolchain does not have RPC support and libtirpc is mandatory.

Instead, disable only the problematic tests, leaving ltp-testsuite usable
with libtirpc.

Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>