interpreter: fix typing for adding tests

We know exactly what type we need, since the interpreter function is
correctly typed and thinly wraps over this. But we didn't even get the
container type correct. :(

mesonbuild/interpreter/interpreter.py

@@ -2216,7 +2216,9 @@ class Interpreter(InterpreterBase, HoldableObject):
                     kwargs['priority'],
                     kwargs['verbose'])
 
-    def add_test(self, node: mparser.BaseNode, args: T.List, kwargs: T.Dict[str, T.Any], is_base_test: bool):
+    def add_test(self, node: mparser.BaseNode,
+                 args: T.Tuple[str, T.Union[build.Executable, build.Jar, ExternalProgram, mesonlib.File]],
+                 kwargs: T.Dict[str, T.Any], is_base_test: bool):
         t = self.make_test(node, args, kwargs)
         if is_base_test:
             self.build.tests.append(t)