selftests: ftrace: Add inter-event hist triggers testcases

    This adds inter-event hist triggers testcases which covers following:
     - create/remove synthetic event
     - disable histogram for synthetic event
     - extended error support
     - field variable support
     - histogram variables
     - histogram trigger onmatch action
     - histogram trigger onmax action
     - histogram trigger onmatch-onmax action
     - simple expression support
     - combined histogram

    Here is the test result.
    === Ftrace unit tests ===
    [1] event trigger - test extended error support [PASS]
    [2] event trigger - test field variable support [PASS]
    [3] event trigger - test inter-event combined histogram trigger [PASS]
    [4] event trigger - test inter-event histogram trigger onmatch action   [PASS]
    [5] event trigger - test inter-event histogram trigger onmatch-onmax action     [PASS]
    [6] event trigger - test inter-event histogram trigger onmax action     [PASS]
    [7] event trigger - test synthetic event create remove  [PASS]

Link: http://lkml.kernel.org/r/e07ef1e72f7bf0f84dc87c9b736d6dc91b4b0b49.1516069914.git.tom.zanussi@linux.intel.com



Signed-off-by: Rajvi Jingar <rajvi.jingar@intel.com>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>