Tests: `gen-ust-nevents`: add syncpoints

Tests: `gen-ust-nevents`: add syncpoints

Adds two sync points:
  `--sync-in-main`: create a file when `gen-ust-nevents` when app is in
  main,
  `--sync-before-first-event`: wait on a file before starting to
  generate any events.

Those two sync points allow for tests to do work when the testapp has
reached main BUT before any events are generated.

This is useful to perform actions on a UST channel once tracing has
started and is active on a particular app.

For example, we want to test a scenario where events are enabled once an
app is already generating other events.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id501c8b373e1a9b43aad6caef11672fe6c30a55a