twister: improve reporting of filtered and built cases

Specify why something was skipped (either filtered or through a ztest
case) and do not report testcases that are only being built as passed,
mark those tests as skipped with the reason specified as "Build only".

Keeping --no-skipped-report for now, although it has no impact.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>