selftests/tracing: Allow some more tests to run in instances

The tests:

trigger-action-hist-xfail.tc
trigger-onchange-action-hist.tc
trigger-snapshot-action-hist.tc
trigger-hist-expressions.tc

can all run in an instance. Test them in an instance as well.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-action-hist-xfail.tc

···
 # SPDX-License-Identifier: GPL-2.0
 # description: event trigger - test inter-event histogram trigger expected fail actions
 # requires: set_event snapshot "snapshot()":README
+# flags: instance
 
 fail() { #msg
     echo $1

tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-onchange-action-hist.tc

···
 # SPDX-License-Identifier: GPL-2.0
 # description: event trigger - test inter-event histogram trigger onchange action
 # requires: set_event "onchange(var)":README ping:program
+# flags: instance
 
 fail() { #msg
     echo $1

tools/testing/selftests/ftrace/test.d/trigger/inter-event/trigger-snapshot-action-hist.tc

···
 # SPDX-License-Identifier: GPL-2.0
 # description: event trigger - test inter-event histogram trigger snapshot action
 # requires: set_event snapshot events/sched/sched_process_fork/hist "onchange(var)":README "snapshot()":README ping:program
+# flags: instance
 
 fail() { #msg
     echo $1

tools/testing/selftests/ftrace/test.d/trigger/trigger-hist-expressions.tc

···
 # SPDX-License-Identifier: GPL-2.0
 # description: event trigger - test histogram expression parsing
 # requires: set_event events/sched/sched_process_fork/trigger events/sched/sched_process_fork/hist error_log "<var1>=<field|var_ref|numeric_literal>":README
+# flags: instance
 
 
 fail() { #msg