selftests/bpf: add pre bpf_prog_test_run_opts() callback for test_loader

selftests/bpf: add pre bpf_prog_test_run_opts() callback for test_loader

When a test case is annotated with __retval tag the test_loader engine
would use libbpf's bpf_prog_test_run_opts() to do a test run of the
program and compare retvals.

This commit allows to perform arbitrary actions on bpf object right
before test loader invokes bpf_prog_test_run_opts(). This could be
used to setup some state for program execution, e.g. fill some maps.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20230420232317.2181776-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>