selftests/bpf: helpers: Add append_tid()

Some tests can't be run in parallel because they use same namespace
names or veth names.

Create an helper that appends the thread ID to a given string. 8
characters are used for it (7 digits + '\0')

Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250131-redirect-multi-v4-1-970b33678512@bootlin.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>