libipt: move event queue handling into its own file

Move the event queue handling code from pt_decoder.[hc] into its own file
pt_event_queue.[hc].  Add struct pt_event_queue to represent a queue of
pending (incomplete) events.

This is mostly just moving code around.  The biggest change is moving
the respective unit tests from check to ptunit.  We want to keep those
tests but get rid of check.  Add tests for pt_evq_find().

Change-Id: I989d1e7330942903667e5dafa34dbe2e52fbd33b
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>