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 +6,9 @@
 
 
 def is_appengine():
-    return "APPENGINE_RUNTIME" in os.environ
+    return "APPENGINE_RUNTIME" in os.environ and not os.environ.get(
+        "SERVER_SOFTWARE", ""
+    ).startswith("testutil/")
 
 
 def is_appengine_sandbox():