selftests/bpf: Add BTF_KIND_FLOAT to btf_dump_test_case_syntax

Check that dumping various floating-point types produces a valid C
code.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210309005649.162480-3-iii@linux.ibm.com