selftests.checkall: Allow 24 tests to fail_once in parallel check

In Travis more tests are failing when running in parallel, but still 20
failed tests in parallel re-ran in series are faster than all tests in
series.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

selftests/checkall

@@ -90,7 +90,7 @@ parallel_selftests() {
             fi
         done
         if [ ${#FAILED_ONCE[@]} -gt 0 ]; then
-            if [ ${#FAILED_ONCE[@]} -le 10 ]; then
+            if [ ${#FAILED_ONCE[@]} -le 24 ]; then
                 echo ${#FAILED_ONCE[@]} failed during parallel execution, trying them in series
                 echo "python -m unittest --failfast ${FAILED_ONCE[@]}"
                 if python -m unittest --failfast ${FAILED_ONCE[@]}; then