selftests/bpf: Add selftest for pointer-to-array-of-struct BTF dump

Bpftool used to issue forward declarations for a struct used as part of
a pointer to array, which is invalid. Add a test to check that the
struct is fully defined in this case:

	@@ -134,9 +134,9 @@
	 	};
	 };

	-struct struct_in_array {};
	+struct struct_in_array;

	-struct struct_in_array_typed {};
	+struct struct_in_array_typed;

	 typedef struct struct_in_array_typed struct_in_array_t[2];

	@@ -189,3 +189,7 @@
	 	struct struct_with_embedded_stuff _14;
	 };

	+struct struct_in_array {};
	+
	+struct struct_in_array_typed {};
	+
	...
	#13/1 btf_dump: syntax:FAIL

Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210319112554.794552-3-jean-philippe@linaro.org