ftracetest: Fix instance test to use proper shell command for pids

The ftracetest instance test used parsing of the "jobs" output to find the
pid of the subshell that is executed previously. But this is not portable to
all major shells that may run these tests. The proper way to get the pid of
the subshell is the shell command "$!". This will return the pid of the
previously executed command. Use that instead, otherwise the test does not
work in all environments.

Link: http://lkml.kernel.org/r/20151211143617.65f4d7a1@gandalf.local.home



Reported-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>