selftests/ftrace: Fix to check fprobe event eneblement

Fix to check the availability of fprobe and kprobes for
add_remove_btfarg.tc.
Only if both kprobe and fprobe are not supported, it should return
"unsupported".

Link: https://lore.kernel.org/all/169024904889.395371.17998733386857387118.stgit@devnote2/

Fixes: 4231f30f

 ("selftests/ftrace: Add BTF arguments test cases")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>