test_all: limit the Zoho tests to just the backend

The free account has a very ungenerous 1000 api calls per day limit
and the full integration test suite breaches that so limit the
integration tests to just the backend.

fstest/test_all/config.yaml

@@ -426,6 +426,8 @@ backends:
  - backend:  "zoho"
    remote:   "TestZoho:"
    fastlist: false
+   tests:
+     - backend
  - backend: "hdfs"
    remote: "TestHdfs:"
    fastlist: false