Add a test demonstrating the opt in nature of args

Args are always opt in for hookimpls regardless of whether the spec or
call defines more then the impl accepts.

Relates to pytest-dev#170

changelog/172.trivial.rst

@@ -0,0 +1 @@
+Add a test exemplifying the opt-in nature of spec defined args.

testing/test_invocations.py

@@ -39,6 +39,34 @@ def hello(self, arg):
     assert comprehensible in str(exc.value)
 
 
+def test_opt_in_args(pm):
+    """Verfiy that two hookimpls with mutex args can serve
+    under the same spec.
+    """
+
+    class Api(object):
+        @hookspec
+        def hello(self, arg1, arg2, common_arg):
+            "api hook 1"
+
+    class LegacyPlugin(object):
+        @hookimpl
+        def hello(self, arg1, common_arg):
+            return old_arg + common_arg
+
+    class Plugin(object):
+        @hookimpl
+        def hello(self, arg2, common_arg):
+            return new_arg + common_arg
+
+    pm.add_hookspecs(Api)
+    pm.register(LegacyPlugin())
+    pm.register(Plugin())
+
+    results = pm.hook.hello(arg1=1, arg2=2, common_arg=0)
+    assert results == [2, 1]
+
+
 def test_call_order(pm):
     class Api(object):
         @hookspec