selftests/exec: recursion-depth: conform test to TAP format output

Conform the layout, informational and status messages to TAP. No
functional change is intended other than the layout of output messages.
While at it, do minor cleanups like move the declarations of the variables
on top of the function.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Link: https://lore.kernel.org/r/20240304155928.1818928-3-usama.anjum@collabora.com
Signed-off-by: Kees Cook <keescook@chromium.org>