t: Move the shebang removal test to its own file

Removing the shebang is something all scripts should do, so move it to a
separate file, so that it can easily and sanely be extended in the
future.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
dh-exec-strip: New sub-command to strip the output

This new sub-command will strip comment, empty and whitespace-only lines
from the output. This script is always run by dh-exec as the last
command in the chain, and cannot be turned off.

Consequently, all kinds of stripping were removed from the other
scripts, as dh-exec-strip will handle that in one place from now on.

Many thanks to Chow Loong Jin <hyperair@debian.org> for noticing the
root problem.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>