testsuite: add content and return-code assertions

testsuite: add content and return-code assertions

Several tests proved only that rsync exited cleanly (or that a file merely
exists), so a no-op/short transfer would pass:

  protected-regular  compare the dst bytes to the source after --inplace.
  00-hello           re-assert one/two were copied on the RSYNC_OLD_ARGS=1
                     env-var path (the explicit --old-args case already did).
  missing            check the dry-run's exit status in test 1.
  mkpath             compare transferred bytes (not just existence) and add a
                     negative control: a transfer WITHOUT --mkpath must fail
                     and create no intermediate path.
  size-filter        compare each kept file's content to its source.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>