Fix plugin interface usage in tests

tests/Composer/Test/Plugin/Fixtures/plugin-v1/Installer/Plugin.php

@@ -9,7 +9,7 @@ class Plugin implements PluginInterface
 {
     public $version = 'installer-v1';
 
-    public function activate(Composer $composer)
+    public function activate()
     {
     }
 }

tests/Composer/Test/Plugin/Fixtures/plugin-v2/Installer/Plugin2.php

@@ -9,7 +9,7 @@ class Plugin2 implements PluginInterface
 {
     public $version = 'installer-v2';
 
-    public function activate(Composer $composer)
+    public function activate()
     {
     }
 }

tests/Composer/Test/Plugin/Fixtures/plugin-v3/Installer/Plugin2.php

@@ -9,7 +9,7 @@ class Plugin2 implements PluginInterface
 {
     public $version = 'installer-v3';
 
-    public function activate(Composer $composer)
+    public function activate()
     {
     }
 }

tests/Composer/Test/Plugin/Fixtures/plugin-v4/Installer/Plugin1.php

@@ -10,7 +10,7 @@ class Plugin1 implements PluginInterface
     public $name = 'plugin1';
     public $version = 'installer-v4';
 
-    public function activate(Composer $composer)
+    public function activate()
     {
     }
 }

tests/Composer/Test/Plugin/Fixtures/plugin-v4/Installer/Plugin2.php

@@ -10,7 +10,7 @@ class Plugin2 implements PluginInterface
     public $name = 'plugin2';
     public $version = 'installer-v4';
 
-    public function activate(Composer $composer)
+    public function activate()
     {
     }
 }