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

tools/testing/selftests/bpf/progs/btf_dump_test_case_syntax.c

@@ -205,6 +205,12 @@ struct struct_with_embedded_stuff {
 	int t[11];
 };
 
+struct float_struct {
+	float f;
+	const double *d;
+	volatile long double *ld;
+};
+
 struct root_struct {
 	enum e1 _1;
 	enum e2 _2;
@@ -219,6 +225,7 @@ struct root_struct {
 	union_fwd_t *_12;
 	union_fwd_ptr_t _13;
 	struct struct_with_embedded_stuff _14;
+	struct float_struct _15;
 };
 
 /* ------ END-EXPECTED-OUTPUT ------ */