Fix build with BUILT_IN_PLUGINS=1

Fix build with BUILT_IN_PLUGINS=1

Changes:

* Make library always depend on its convenience libraries. Since the
  library is not linked with --whole-archive in the CLI when
  BUILT_IN_PLUGINS=1 (like plugins are), it's okay for it to include
  convenience symbols.

* Do not build tests/lib/test_plugin when BUILT_IN_PLUGINS=1

* Do not build tests/plugins when BUILT_IN_PLUGINS=1

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