tests: add a test for spack debug command

lib/spack/llnl/util/lang.py

@@ -555,6 +555,9 @@ def instance(self):
     def __getattr__(self, name):
         return getattr(self.instance, name)
 
+    def __getitem__(self, name):
+        return self.instance[name]
+
     def __str__(self):
         return str(self.instance)
 
@@ -571,6 +574,9 @@ def __init__(self, ref_function):
     def __getattr__(self, name):
         return getattr(self.ref_function(), name)
 
+    def __getitem__(self, name):
+        return self.ref_function()[name]
+
     def __str__(self):
         return str(self.ref_function())