selftests: drv-net: rss_ctx: skip tests which need multiple contexts cleanly

There's no good API to check how many contexts device supports.
But initial tests sense the context count already, so just store
that number and skip tests which we know need more.

Link: https://patch.msgid.link/20250206235334.1425329-7-kuba@kernel.org


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