tests: Fix DeprecationWarning in SRv6 L3VPN topotest

tests: Fix DeprecationWarning in SRv6 L3VPN topotest

Fix the following warning:

tests/topotests/bgp_srv6l3vpn_sid/test_bgp_srv6l3vpn_sid.py:42
  /media/SharedUTM/workspace/frr/tests/topotests/bgp_srv6l3vpn_sid/test_bgp_srv6l3vpn_sid.py:42: DeprecationWarning: invalid escape sequence '\ '

In test_bgp_srv6l3vpn_sid.py we have a comment containing some '\'
characters. Python mistakenly tries to interpret such "\" characters
as escape sequences, which leads to the above warning.

Let's tell Python to treat the comment as a raw string,
so that it simply treats backslashes as literal characters rather than
escape sequences.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>