make sure tests and benchmarks use correct backend

[nturgut]

Description

This PR makes sure that rendering backend is set correctly for tests. This will enable us to re-merge: flutter/engine#23090
When the backend is auto, html backends tests will still use the correct backend.

Related Issues

#72377

Tests

These are changes related to test options.

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[flutter-dashboard]

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[surda19-jin]

End