Runner config: disable diagnosticMessages

Useful when hanging, but noisy when not. Disable for now, but leave in config for when needed readily.

StackExchange.Redis.Tests/xunit.runner.json

 {
   "methodDisplay": "classAndMethod",
   "maxParallelThreads": 3,
-  "diagnosticMessages": true,
+  "diagnosticMessages": false,
   "longRunningTestSeconds": 60
 }
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