selftests: drv-net: Make command requirements explicit

Make require_cmd() calls explicit about whether commands are needed
locally, remotely, or both.
Since require_cmd() defaults to local=True, tests should explicitly set
local=False when commands are only needed remotely.

- socat: Set local=False since it's only needed on remote hosts.
- iperf3: Use single call with both local=True and remote=True since
  it's needed on both hosts.

This avoids unnecessary test failures when commands are missing locally
but available remotely where actually needed, and consolidates a
duplicate require_cmd() call into single call that checks both hosts.

Fixes: 0d0f4174 ("selftests: drv-net: add a simple TSO test")
Fixes: f1e68a1a ("selftests: drv-net: add require_XYZ() helpers for validating env")
Fixes: c76bab22 ("selftests: drv-net: rss_input_xfrm: Check test prerequisites before running")
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Link: https://patch.msgid.link/20250723135454.649342-3-gal@nvidia.com


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