selftests/sched_ext: Return non-zero exit code on test failure

selftests/sched_ext: Return non-zero exit code on test failure

runner.c always returned 0 regardless of test results.  The kselftest
framework (tools/testing/selftests/kselftest/runner.sh) invokes the runner
binary and treats a non-zero exit code as a test failure; with the old
code, failed sched_ext tests were silently hidden from the parent harness
even though individual "not ok" TAP lines were emitted.

Return 1 when at least one test failed, 0 when all tests passed or were
skipped.

Signed-off-by: zhidao su <suzhidao@xiaomi.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>