selftests/bpf: Adjust BPF selftest for xdp_adjust_tail

Current selftest for BPF-helper xdp_adjust_tail only shrink tail.
Make it more clear that this is a shrink test case.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/158945350058.97035.17280775016196207372.stgit@firesoul

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

@@ -2,9 +2,9 @@
 #include <test_progs.h>
 #include <network_helpers.h>
 
-void test_xdp_adjust_tail(void)
+void test_xdp_adjust_tail_shrink(void)
 {
-	const char *file = "./test_adjust_tail.o";
+	const char *file = "./test_xdp_adjust_tail_shrink.o";
 	struct bpf_object *obj;
 	char buf[128];
 	__u32 duration, retval, size;
@@ -28,3 +28,8 @@ void test_xdp_adjust_tail(void)
 	      err, errno, retval, size);
 	bpf_object__close(obj);
 }
+
+void test_xdp_adjust_tail(void)
+{
+	test_xdp_adjust_tail_shrink();
+}