[DiffRunner integration] Support disabling diff tools AND communicate with DiffEngineTray

[espenrl]

Hi, I want to avoid diff tools being opened automatically, and use DiffEngineTray for managing the verify tests.

DiffRunner logic

When DiffRunner.Disabled is set true then DiffRunner will still communicate with DiffEngineTray. And the diff tools will be disabled.

This is the logic wanted.

Verify logic

Reads DiffRunner.Disabled and doesn't ever call DiffRunner.Launch* methods. Thus no communication with DiffEngineTray.

https://github.com/VerifyTests/Verify/blob/fd75b393729b8b12471f02dfc6a365c875fc5e4c/src/Verify/Verifier/VerifyEngine.cs#L14-L16

Workaround

I tried DiffRunner.MaxInstancesToLaunch(0), but failed due to guard against zero.

Environment.SetEnvironmentVariable("DiffEngine_MaxInstances", "0") works as it is not guarded against zero.

[SimonCropp]

MaxInstancesToLaunch(0) is now allowed

[espenrl]

Thanks @SimonCropp