Investigate feature flags in test resources subs

We may have accumulated feature flags in the Azure SDK Test Resources subscription. When chatting with @weshaggard about an intermittent issue arising in C++ attestation tests, it was discovered that `Get-AzProviderFeature` returns a long list of what may be feature flags.

Since we want to make the Test Resources subscription as free of special configuration as possible, the accumulation of these flags may be operating against that goal.

If these are feature flags we should consider:
1. Removing those feature flags
2. Setting the feature flags in the test resources `Preview` subscription
3. Ensuring that tests are updated appropriately to start using the `Preview` subscription

To see these flags:

```pwsh
Connect-AzAccount
Set-AzContext -SubscriptionId ''
Get-AzProviderFeature
```

It is expected that this list produced by this command would have very few if any results for a subscription that was free of special configuration.