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>

Author: Zheng Yejian

tools/testing/selftests/ftrace/test.d/event/toplevel-enable.tc

@@ -1,7 +1,8 @@
 #!/bin/sh
 # SPDX-License-Identifier: GPL-2.0
 # description: event tracing - enable/disable with top level files
-# requires: available_events set_event events/enable
+# requires: set_event events/enable
+# flags: instance
 
 do_reset() {
     echo > set_event