pw_unit_test: Fix first expected RPC response
pw_unit_test: Fix first expected RPC response

This updates the RPC unit testing Python code to expect a test_run_start
message as the first RPC response, and trigger event handlers when it is
received.

Change-Id: I510335b672ef24ef0bf77411f6072bb1735c2204
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/32124
Reviewed-by: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>

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