deps module utils: unit tests + minor improvement (#9179)

* deps module utils: unit tests + minor improvement

* deps.clear() calls dict.clear() instead of creating new dict

* add changelog frag

plugins/module_utils/deps.py

@@ -96,3 +96,7 @@ def validate(module, spec=None):
 
 def failed(spec=None):
     return any(_deps[d].failed for d in _select_names(spec))
+
+
+def clear():
+    _deps.clear()