tests/bpf.c: convert TEST_BPF_ macro to a function

tests/bpf.c: convert TEST_BPF_ macro to a function

As it looks like there's no compelling reason to keep it as a macro and
leaving it so quite complicates modification of the routine and makes
them error-prone.

* tests/bpf.c (TEST_BPF_): Convert it to...
(test_bpf): ...this.  Rename cmd_ to cmd, cmd_str_ to cmd_str,
init_first_ to init_first, print_first_ to print_first, init_attr_
to init_attr, print_attr_ to print_attr.
(TEST_BPF): Call test_bpf_ instead of TEST_BPF_.