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

tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c

@@ -208,11 +208,18 @@ static void test_func_map_prog_compatibility(void)
 
 void test_fexit_bpf2bpf(void)
 {
+	if (test__start_subtest("target_no_callees"))
 		test_target_no_callees();
+	if (test__start_subtest("target_yes_callees"))
 		test_target_yes_callees();
+	if (test__start_subtest("func_replace"))
 		test_func_replace();
+	if (test__start_subtest("func_replace_verify"))
 		test_func_replace_verify();
+	if (test__start_subtest("func_sockmap_update"))
 		test_func_sockmap_update();
+	if (test__start_subtest("func_replace_return_code"))
 		test_func_replace_return_code();
+	if (test__start_subtest("func_map_prog_compatibility"))
 		test_func_map_prog_compatibility();
 }