Add bt_plugin test

Add bt_plugin test

This new test proves that the bt_plugin subsystem works as expected.

There are three Babeltrace plugins built in
`tests/lib/test-plugin-plugins`. They are:

* `minimal`: Minimal, valid plugin with no component classes.
* `sfs`: Valid plugin with a source, a filter, and a sink component
  class.
* `invalid`: Invalid plugin (missing name).

Verified in this test:

* Behaviour of bt_plugin_*() functions with non-existing paths, invalid
  arguments, etc.

* bt_plugin_create_from_file() can load a valid plugin and we can access
  all its properties.

* The initialization and exit functions of a plugin are called when
  expected.

* We can access the component classes of a plugin which provides some.

* We can still create a component from a component class provided by
  a plugin after the associated plugin object is destroyed, that is, the
  associated shared library handle is not closed until it is known that
  no more user code found in the loaded object will ever be executed
  again in the future.

* bt_plugin_create_all_from_dir() works as expected.

As of this patch, Valgrind shows for this test:

    ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>