Rename "observatoryUri" to "vmServiceUri" for test.startedProcess

[helomri]

This is due to the removal of Observatory support from Dart SDKs and Flutter which made the tests never unpause on recent versions of Flutter. This fixes #8233.

The URI pointing to Observatory is no longer given in test.startedProcess. Now, the URI to the VM Service for DevTools is present and can be used as a drop-in replacement to achieve the same functionality.

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[jwren]

Change looks good, the follow up here if you can would be to add a check for the old id if this fails. Also to add documentation to the issue.

[helomri]

@jwren Done. It's wise to keep both checks as 3.34 hasn't landed on stable yet and keeping backwards compatibility in this case is not that hard.

[jwren]

@helomri Thanks for updating, I'll land it when the presubmits pass.

[huycozy]

Thank @helomri for the fix. Hope this will land on stable soon!