tests: Fix warnings in `bgp_srv6l3vpn_to_bgp_vrf`

tests: Fix warnings in `bgp_srv6l3vpn_to_bgp_vrf`

When performing the `bgp_srv6l3vpn_to_bgp_vrf` topotest, the following
warnings are observed:

```
2024-07-21 08:01:51,390 WARNING: r1: Router(r1): proc failed: rc 127 pid 52974
	args: /usr/bin/nsenter --mount=/proc/52322/ns/mnt --net=/proc/52322/ns/net --uts=/proc/52322/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/r1/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/r1/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:51,522 WARNING: r2: Router(r2): proc failed: rc 127 pid 52984
	args: /usr/bin/nsenter --mount=/proc/52397/ns/mnt --net=/proc/52397/ns/net --uts=/proc/52397/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/r2/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/r2/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:51,632 WARNING: ce1: Router(ce1): proc failed: rc 127 pid 52994
	args: /usr/bin/nsenter --mount=/proc/52472/ns/mnt --net=/proc/52472/ns/net --uts=/proc/52472/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce1/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce1/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:51,757 WARNING: ce2: Router(ce2): proc failed: rc 127 pid 53004
	args: /usr/bin/nsenter --mount=/proc/52547/ns/mnt --net=/proc/52547/ns/net --uts=/proc/52547/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce2/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce2/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:51,878 WARNING: ce3: Router(ce3): proc failed: rc 127 pid 53014
	args: /usr/bin/nsenter --mount=/proc/52622/ns/mnt --net=/proc/52622/ns/net --uts=/proc/52622/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce3/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce3/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:51,997 WARNING: ce4: Router(ce4): proc failed: rc 127 pid 53024
	args: /usr/bin/nsenter --mount=/proc/52697/ns/mnt --net=/proc/52697/ns/net --uts=/proc/52697/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce4/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce4/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:52,109 WARNING: ce5: Router(ce5): proc failed: rc 127 pid 53034
	args: /usr/bin/nsenter --mount=/proc/52772/ns/mnt --net=/proc/52772/ns/net --uts=/proc/52772/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce5/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce5/setup.sh: No such file or directory
	stderr: *empty*
2024-07-21 08:01:52,225 WARNING: ce6: Router(ce6): proc failed: rc 127 pid 53044
	args: /usr/bin/nsenter --mount=/proc/52847/ns/mnt --net=/proc/52847/ns/net --uts=/proc/52847/ns/uts -F /bin/bash -c /bin/bash /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce6/setup.sh
	stdout: /bin/bash: /media/workspace/frr/tests/topotests/bgp_srv6l3vpn_to_bgp_vrf/ce6/setup.sh: No such file or directory
	stderr: *empty*
````

This occurs because the topotest attempts to execute the `setup.sh`
file, and the file does not exist.

Let's fix the issue by checking if the `setup.sh` file exists and
executing it only if it does.

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