Load only tested plugins when invoked with -test

By default, a clean settings path is used for test environment.

All the default plugins are loaded, although they're not needed.

This change significantly improves loading time for tests.

Change-Id: I24254f3e538e3f0e6d233d0989738dc1ce238209
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

doc/api/plugin-metadata.qdoc

@@ -199,8 +199,9 @@
         \row
             \li Type
             \li String
-            \li Optional. Value \c Required or \c Optional. Defines if the dependency is
-                a hard requirement or optional. Defaults to \c{Required}.
+            \li Optional. Value \c Required, \c Optional, or \c Test. Defines if the dependency is
+                a hard requirement, optional, or required for running the plugin's tests.
+                Defaults to \c{Required}.
     \endtable
 
     \section3 Optional Dependencies
@@ -223,6 +224,18 @@
     ExtensionSystem::PluginManager::getObjectByClassName(), and use QMetaObject functions to call
     functions on it.
 
+    \section3 Test Dependencies
+
+    When the user runs the application with the \c{-test} command line argument, only
+    the specified plugins and their dependencies are loaded. This is done in order to
+    speed up the execution of tests by avoiding the loading of unneeded plugins.
+
+    A plugin can specify additional dependencies that are required for running its
+    tests, but not for its normal execution, by declaring dependencies with
+    \c {"Type" : "Test"}. Test dependencies are force loaded, and do not affect load order.
+
+    This type of dependency is not transitive.
+
     \section2 Command Line Arguments
 
     Plugins can register command line arguments that the user can give