selftests/livepatch: update test-ftrace.sh for deprecated ftrace_enabled

kernel.ftrace_enabled=0 is now refused on kernels that deprecate the
knob, so the old disable/reload flow no longer applies there. Probe
for this with ftrace_disable_supported() and keep the full original
scenario (disable, fail to load a livepatch, re-enable, load, confirm
disable is refused while loaded) on kernels where it still works;
otherwise just confirm the write is refused.

Link: https://patch.msgid.link/20260806153000.4184871-3-andrey.grodzovsky@crowdstrike.com
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Joe Lawrence <joe.lawrence@redhat.com>
Assisted-by: Claude:claude-sonnet-5
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>