Update Aspire.Hosting.Testing doc to cover new changes in 8.1

[DamianEdwards]

• Using ResourceNotificationService.WaitForResourceAsync in tests to avoid sending requests to resources before they've transitioned to the "Running" state (Add ResourceNotificationService.WaitForResourceAsync aspire#4445)
• Detail how resource console logs are redirected to the DistributedApplication host logs by default when using DistributedApplicationTestingBuilder in 8.1
• Document how to use FakeLogger from Microsoft.Extensions.Diagnostics.Testing in tests to assert the logs emitted from resources, e.g. to fail a test if a resource logs any errors.
• Document the new GetEnvironmentVariableValuesAsync method in Add GetEnvironmentVariableValuesAsync method to ResourceExtensions aspire#4530.

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Associated WorkItem - 308093