ftracetest: Use proper logic to find process PID

Half of the test in instance-event.tc was updated to use $! to find the PID
of the previous background process that was launched, but the second part of
the test still used the parsing of "jobs", which does not work on all shells
like $! does.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

tools/testing/selftests/ftrace/test.d/instances/instance-event.tc

@@ -92,28 +92,23 @@ instance_slam() {
 }
 
 instance_slam &
-x=`jobs -l`
-p1=`echo $x | cut -d' ' -f2`
+p1=$!
 echo $p1
 
 instance_slam &
-x=`jobs -l | tail -1`
-p2=`echo $x | cut -d' ' -f2`
+p2=$!
 echo $p2
 
 instance_slam &
-x=`jobs -l | tail -1`
-p3=`echo $x | cut -d' ' -f2`
+p3=$!
 echo $p3
 
 instance_slam &
-x=`jobs -l | tail -1`
-p4=`echo $x | cut -d' ' -f2`
+p4=$!
 echo $p4
 
 instance_slam &
-x=`jobs -l | tail -1`
-p5=`echo $x | cut -d' ' -f2`
+p5=$!
 echo $p5
 
 ls -lR >/dev/null