selftests: bpf: enable hi32 randomization for all tests

The previous libbpf patch allows user to specify "prog_flags" to bpf
program load APIs. To enable high 32-bit randomization for a test, we need
to set BPF_F_TEST_RND_HI32 in "prog_flags".

To enable such randomization for all tests, we need to make sure all places
are passing BPF_F_TEST_RND_HI32. Changing them one by one is not
convenient, also, it would be better if a test could be switched to
"normal" running mode without code change.

Given the program load APIs used across bpf selftests are mostly:
  bpf_prog_load:      load from file
  bpf_load_program:   load from raw insns

A test_stub.c is implemented for bpf seltests, it offers two functions for
testing purpose:

  bpf_prog_test_load
  bpf_test_load_program

The are the same as "bpf_prog_load" and "bpf_load_program", except they
also set BPF_F_TEST_RND_HI32. Given *_xattr functions are the APIs to
customize any "prog_flags", it makes little sense to put these two
functions into libbpf.

Then, the following CFLAGS are passed to compilations for host programs:
  -Dbpf_prog_load=bpf_prog_test_load
  -Dbpf_load_program=bpf_test_load_program

They migrate the used load APIs to the test version, hence enable high
32-bit randomization for these tests without changing source code.

Besides all these, there are several testcases are using
"bpf_prog_load_attr" directly, their call sites are updated to pass
BPF_F_TEST_RND_HI32.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>