test_nptl: fix expected result for tst-clock2 test
tst-clock2 should return 0 when _SC_THREAD_CPUTIME option isn't
available, instead of treating it as an error.
Further set the expected ret value as 0 avoiding to hide any real failures
in case of THREAD_CPUTIME feature available.
Signed-off-by: Filippo Arcidiacono <filippo.arcidiacono@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>