Auto merge of #29237 - servo:simplify-wpt-run, r=jdm

Simplify Python code for running WPT tests

Combine `run.py` and `update.py` into `servowpt.py` in order to allow them to share code. Import them directly into the mach script to avoid having to call `compile` and `exec` on the code. This makes it clearer how they are executed. In addition, move all of the setup into `setupwpt.py` to avoid differences between tests executed via mach and not. Finally, be more ambitious when detecting the build to use. If none was specified, try to use the one that exists between "release" and "debug."

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<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they only change a small script behavior.

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