selftests/bpf: Turn fexit_bpf2bpf into test with subtests

There are clearly 4 subtests, so make it official.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200903203542.15944-12-andriin@fb.com