[Tests fixed!] Process DeviceCommands in simulated devices to ensure sync requests are correctly processed

[Zee2]

Overview

This PR will fix play-mode tests in batch mode (i.e., CI)!

• Made SimulatedControllers implement ExecuteCommand to receive Sync commands and report success, fixing the focus-lost-device-reset bug
• [x] Make SimulatedCamera implement the above ExecuteCommand method to report success on sync commands
• Changed our test harness to use the InputTestFixture
• Added com.unity.inputsystem to testables
• Manually registered XRI custom interactions/fallbackcomposites that were not registered in the custom input backend that the TestFixture uses, which was preventing us from using the InputTestFixture
• Fixed edge case in cursor magnetism script that was referencing colliders after they had been destroyed
• [x] Find out why roughly half of the tests continue to fail despite the input devices + interactors working correctly

Breaking: All references to "Simulated Camera" replaced with Simulated HMD for accuracy.

[Zee2]

/azp run

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