selftests/ftrace: Fix same probe error test

The "same probe" selftest that tests that adding the same probe fails
doesn't add the same probe and passes, which fails the test.

Fixes: b78b94b8 ("selftests/ftrace: Update kprobe event error testcase")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>