is_appengine=False in testbed (like it used to)

Whether testbed tests "are appengine" is debatable, but historically
this function has returned False in testbed tests. This behavior was
inadvertently (and unnecessarily) changed in PR urllib3#1704.  This commit
undoes that regression for testbed tests.

src/urllib3/contrib/_appengine_environ.py

@@ -6,7 +6,7 @@
 
 
 def is_appengine():
-    return "APPENGINE_RUNTIME" in os.environ
+    return is_local_appengine() or is_prod_appengine()
 
 
 def is_appengine_sandbox():
@@ -20,15 +20,15 @@ def is_appengine_sandbox():
 
 
 def is_local_appengine():
-    return is_appengine() and os.environ.get("SERVER_SOFTWARE", "").startswith(
-        "Development/"
-    )
+    return "APPENGINE_RUNTIME" in os.environ and os.environ.get(
+        "SERVER_SOFTWARE", ""
+    ).startswith("Development/")
 
 
 def is_prod_appengine():
-    return is_appengine() and os.environ.get("SERVER_SOFTWARE", "").startswith(
-        "Google App Engine/"
-    )
+    return "APPENGINE_RUNTIME" in os.environ and os.environ.get(
+        "SERVER_SOFTWARE", ""
+    ).startswith("Google App Engine/")
 
 
 def is_prod_appengine_mvms():