tests: add mechanism to change Weston's behavior when running certain tests

There are some specific cases in which we need Weston to
behave differently when running in the test suite. This
adds a new API to allow the tests to select these behaviors.

For instance, in the DRM backend we plan to add a writeback
connector screenshooter. In case it fails for some
reason, it should fallback to the renderer screenshooter
that all other backends use. But if we add a test to
ensure the correctness of the writeback screenshooter,
we don't want it to fallback to the renderer one, we
want it to fail. With this new API we can choose to
disable the fallback behavior specifically for this test.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>

libweston/compositor.c

@@ -7327,6 +7327,55 @@ debug_scene_graph_cb(struct weston_log_subscription *sub, void *data)
 	weston_log_subscription_complete(sub);
 }
 
+/** Init the compositor testsuite data
+ *
+ * The struct weston_testsuite_data contains two members:
+ *
+ *  1. The struct weston_testsuite_quirks, which can be used by the tests to
+ *  change certain behavior of Weston when running these tests.
+ *
+ *  2. The struct wet_testsuite_data member, which can be used by the test suite
+ *  of projects that uses libweston in order to give arbitrary test data to the
+ *  compositor.
+ *
+ * This function can be called at most once per compositor instance, just after
+ * creating the weston_compositor object and never again. This happens because
+ * changing the quirks after e.g. loading the backend is not going to work,
+ * there are certain behaviors that need to be set up before this point.
+ *
+ * \param ec The weston compositor.
+ * \param test_data The testsuite data.
+ *
+ * \ingroup compositor
+ * \sa weston_compositor_get_test_data
+ */
+WL_EXPORT void
+weston_compositor_test_data_init(struct weston_compositor *ec,
+				 const struct weston_testsuite_data *test_data)
+{
+	assert(ec->backend == NULL);
+	ec->test_data = *test_data;
+}
+
+/** Retrieve testsuite data from compositor
+ *
+ * The testsuite data can be defined by the test suite of projects that uses
+ * libweston and given to the compositor at the moment of its creation. This
+ * function should be used when we need to retrieve the testsuite private data
+ * from the compositor.
+ *
+ * \param ec The weston compositor.
+ * \return The testsuite data.
+ *
+ * \ingroup compositor
+ * \sa weston_compositor_test_data_init
+ */
+WL_EXPORT void *
+weston_compositor_get_test_data(struct weston_compositor *ec)
+{
+	return ec->test_data.test_private_data;
+}
+
 /** Create the compositor.
  *
  * This functions creates and initializes a compositor instance.