premiumizeme: attempt to fix integration tests

This tries to fix the integration tests by only allowing one
premiumizeme test to run at once, in the hope it will stop rclone
hitting the rate limits and breaking the tests.

See: #5734

fstest/test_all/config.yaml

@@ -296,6 +296,7 @@ backends:
  - backend:  "premiumizeme"
    remote:   "TestPremiumizeMe:"
    fastlist: false
+   oneonly:  true
  - backend:  "putio"
    remote:   "TestPutio:"
    fastlist: false