backend/backends: Fix type assertions in backend

Test dependencies can be of type CustomTargetIndex,
similar to other targets.

mesonbuild/backend/backends.py

@@ -1171,7 +1171,7 @@ class Backend:
 
     def determine_windows_extra_paths(
             self, target: T.Union[build.BuildTarget, build.CustomTarget, build.CustomTargetIndex, programs.ExternalProgram, mesonlib.File, str],
-            extra_bdeps: T.Sequence[T.Union[build.BuildTarget, build.CustomTarget]]) -> T.List[str]:
+            extra_bdeps: T.Sequence[T.Union[build.BuildTarget, build.CustomTarget, build.CustomTargetIndex]]) -> T.List[str]:
         """On Windows there is no such thing as an rpath.
 
         We must determine all locations of DLLs that this exe
@@ -1231,7 +1231,7 @@ class Backend:
             exe_wrapper = self.environment.get_exe_wrapper()
             machine = self.environment.machines[exe.for_machine]
             if machine.is_windows() or machine.is_cygwin():
-                extra_bdeps: T.List[T.Union[build.BuildTarget, build.CustomTarget]] = []
+                extra_bdeps: T.List[T.Union[build.BuildTarget, build.CustomTarget, build.CustomTargetIndex]] = []
                 if isinstance(exe, build.CustomTarget):
                     extra_bdeps = list(exe.get_transitive_build_target_deps())
                 extra_paths = self.determine_windows_extra_paths(exe, extra_bdeps)
@@ -1428,7 +1428,7 @@ class Backend:
                     continue
                 result[arg.get_id()] = arg
             for dep in t.depends:
-                assert isinstance(dep, (build.CustomTarget, build.BuildTarget))
+                assert isinstance(dep, (build.CustomTarget, build.BuildTarget, build.CustomTargetIndex))
                 result[dep.get_id()] = dep
         return result