selftests/bpf: Add test for libbpf_bpf_map_type_str

This change adds a test for libbpf_bpf_map_type_str. The test retrieves
all variants of the bpf_map_type enumeration using BTF and makes sure
that the function under test works as expected for them.

Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220523230428.3077108-6-deso@posteo.net