selftests: net: py: don't default to shell=True

Overhead of using shell=True is quite significant.
Micro-benchmark of running ethtool --help shows that
non-shell run is 2x faster.

Runtime of the XDP tests also shows improvement:
this patch: 2m34s 2m21s 2m18s 2m18s
    before:     2m54s 2m36s 2m34s

Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250830184317.696121-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>