selftests: bpf: Check that PROG_TEST_RUN repeats as requested

Extend a simple prog_run test to check that PROG_TEST_RUN adheres
to the requested repetitions. Convert it to use BPF skeleton.

Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210303101816.36774-5-lmb@cloudflare.com