selftests: net: use setup_ns for SRv6 tests and remove rp_filter configuration

selftests: net: use setup_ns for SRv6 tests and remove rp_filter configuration

Some SRv6 tests manually set up network namespaces and disable rp_filter.
Since the setup_ns library function already handles rp_filter configuration,
convert these SRv6 tests to use setup_ns and remove the redundant rp_filter
settings.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20250508081910.84216-5-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>