plugins: refactor the tests for dynamic plugins

This adapts the test to the new 'plugin' command: no more sleeping,
since we are synchronous !

This tests the timeout by increasing the 'slowinit' plugin sleep
duration at init reception.

This adds a broken plugin to make sure we won't crash because of a
misbehaving plugin (unmet dependency is the most common case).

tests/plugins/slow_init.py

@@ -8,7 +8,7 @@ plugin = Plugin()
 @plugin.init()
 def init(options, configuration, plugin):
     plugin.log("slow_init.py initializing {}".format(configuration))
-    time.sleep(1)
+    time.sleep(2)
 
 
 plugin.run()