This change creates a base class for the Servo process executors, to
handle shared functionality. The only thing that hasn't moved there yet
is the actual process execution, which can happen in a followup change.
The main motivation behind this change is consistently handling
`on_evironment_change` which is used to handle changes to the `prefs`
value stored in `__dir__.ini`. Inherited `prefs` (thos in `__dir__.ini`)
aren't passed when creating tests for reference HTML. This change takes
a similar appraoch to Gecko, which just listens to
`on_environment_change` to note when these prefs change.
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