selftests/sched_ext: Improve runner error reporting for invalid arguments

selftests/sched_ext: Improve runner error reporting for invalid arguments

Report an error for './runner foo' (positional arg instead of -t) and
for './runner -t foo' when the filter matches no tests. Previously both
cases produced no error output.

Pre-scan the test list before the main loop so the error is reported
immediately, avoiding spurious SKIP output from '-s' when no tests
match.

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>