selftests/ftrace: Add test to remove instance with active event triggers

Add a test to ensure we clean up properly when removing an instance
with active event triggers.

Link: http://lkml.kernel.org/r/c479465b2009397708d6c52c8561e1523c22cd31.1494956770.git.naveen.n.rao@linux.vnet.ibm.com



Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>