selftests/ftrace: Add a testcase for multiple fprobe events

Add a testcase for multiple fprobe events on the same function
so that it clears ftrace hash map correctly when removing the
events.

Link: https://lore.kernel.org/all/177669370353.132053.16801520791509406141.stgit@mhiramat.tok.corp.google.com/

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>