selftests/bpf: Move vxlan tunnel testcases to test_progs

Move vxlan tunnel testcases from test_tunnel.sh to test_progs.
And add vxlan tunnel source testcases also. Other tunnel testcases
will be moved to test_progs step by step in the future.
Rename bpf program section name as SEC("tc") because test_progs
bpf loader could not load sections with name SEC("gre_set_tunnel").
Because of this, add bpftool to load bpf programs in test_tunnel.sh.

Signed-off-by: Kaixi Fan <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: Kaixi Fan