selftests/net: fix reinitialization of TEST_PROGS in net self tests.

Assinging will drop all previous tests.

Fixes: b690842d

 ("selftests/net: test l2 tunnel TOS/TTL inheriting")
Signed-off-by: Adel Abouchaev <adel.abushaev@gmail.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20220824184351.3759862-1-adel.abushaev@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>