bpf: selftests: test_tunnel: Setup fresh topology for each subtest

This helps with determinism b/c individual setup/teardown prevents
leaking state between different subtests.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/0fb59fa16fb58cca7def5239df606005a3e8dd0e.1702593901.git.dxu@dxuuu.xyz


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