[llvm] [docs] Expand HowToAddABuilder with guidance on testing locally (PR #115024)

[David Spickett via llvm-commits]

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@@ -179,6 +179,74 @@ Here are the steps you can follow to do so:
    buildbot.tac file to change the port number from 9994 to 9990 and start it
    again.
 
+Testing a Builder Config Locally
+================================
+
+It's possible to test a builder running against a local version of LLVM's
+buildmaster configuration. This can be helpful to allow quickly identifying
+and iterating over fixes to any issues in either the changes that introduce
+the new builder, or the machine configuration for your worker (preinstalled
+packages etc). A buildmaster launched in this "local testing" mode will bind
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DavidSpickett wrote:

On the grounds that folks should read and (to themselves) agree with each point in turn.

https://github.com/llvm/llvm-project/pull/115024