selftests/bpf: Close the file descriptor to avoid resource leaks

Static analysis found an issue in bench_htab_mem.c and sk_assign.c

cppcheck output before this patch:
tools/testing/selftests/bpf/benchs/bench_htab_mem.c:284:3: error: Resource leak: fd [resourceLeak]
tools/testing/selftests/bpf/prog_tests/sk_assign.c:41:3: error: Resource leak: tc [resourceLeak]

cppcheck output after this patch:
No resource leaks found

Fix the issue by closing the file descriptors fd and tc.

Signed-off-by: Malaya Kumar Rout <malayarout91@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250421174405.26080-1-malayarout91@gmail.com