selftests/bpf: Add BTF_KIND_FLOAT to the existing deduplication tests

Check that floats don't interfere with struct deduplication, that they
are not merged with another kinds and that floats of different sizes are
not merged with each other.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226202256.116518-9-iii@linux.ibm.com