selftests/bpf: Switch test_maps to test_progs' test.h format

Make test_maps use tests.h header format consistent with the one used by
test_progs, to facilitate unification.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191016060051.2024182-4-andriin@fb.com