selftests/net: convert srv6_end_dt6_l3vpn_test.sh to run it in unique namespace

As the name \${rt-${rt}} may make reader confuse, convert the variable
hs/rt in setup_rt/hs to hid, rid. Here is the test result after conversion.

 ]# ./srv6_end_dt6_l3vpn_test.sh

 ################################################################################
 TEST SECTION: IPv6 routers connectivity test
 ################################################################################

     TEST: Routers connectivity: rt-1 -> rt-2                            [ OK ]

     TEST: Routers connectivity: rt-2 -> rt-1                            [ OK ]
 ...

     TEST: Hosts isolation: hs-t200-4 -X-> hs-t100-2                     [ OK ]

 Tests passed:  18
 Tests failed:   0

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-5-liuhangbin@gmail.com


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