selftests/ftrace: Test toplevel-enable for instance

'available_events' is actually not required by
'test.d/event/toplevel-enable.tc' and its Existence has been tested in
'test.d/00basic/basic4.tc'.

So the require of 'available_events' can be dropped and then we can add
'instance' flag to test 'test.d/event/toplevel-enable.tc' for instance.

Test result show as below:
# ./ftracetest test.d/event/toplevel-enable.tc
=== Ftrace unit tests ===
[1] event tracing - enable/disable with top level files [PASS]
[2] (instance) event tracing - enable/disable with top level files [PASS]

# of passed: 2
# of failed: 0
# of unresolved: 0
# of untested: 0
# of unsupported: 0
# of xfailed: 0
# of undefined(test bug): 0

Link: https://lore.kernel.org/r/20230509203659.1173917-1-zhengyejian1@huawei.com
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>