ftrace/selftests: workaround cgroup RT scheduling issues

wakeup_rt.tc and wakeup.tc tests in tracers/ subdirectory
fail due to the chrt command returning:

 chrt: failed to set pid 0's policy: Operation not permitted.

To work around this, temporarily disable grout RT scheduling
during ftracetest execution.  Restore original value on
test run completion.  With these changes in place, both
tests consistently pass.

Fixes: c575dea2 ("selftests/ftrace: Add wakeup_rt tracer testcase")
Fixes: c1edd060 ("selftests/ftrace: Add wakeup tracer testcase")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>