selftests/bpf: Move test_progs helpers to testing_helpers object

selftests/bpf: Move test_progs helpers to testing_helpers object

Moving test_progs helpers to testing_helpers object so they can be
used from test_verifier in following changes.

Also adding missing ifndef header guard to testing_helpers.h header.

Using stderr instead of env.stderr because un/load_bpf_testmod helpers
will be used outside test_progs. Also at the point of calling them
in test_progs the std files are not hijacked yet and stderr is the
same as env.stderr.

Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230515133756.1658301-4-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>