Enable DevTestLabs tests

Currently there are five tests in the DevTestLabs test suite, but none of them are running as a part of our current build:

- `TestAzureRmDtlVMsPerLabPolicy`
- `TestAzureRmDtlVMsPerUserPolicy`
- `TestAzureRmDtlAllowedVMSizesPolicy`
- `TestAzureRmDtlAutoShutdownPolicy`
- `TestAzureRmDtlAutoStartPolicy`

There is no proper test setup for these tests (_i.e._, resource group(s) aren't created, variables used aren't instantiated), so it is difficult to re-record these tests. The work needs to be done to enable these tests and ensure that the proper test setup is added, and then the test assembly should be referenced in the [`TestMappings.json`](https://github.com/Azure/azure-powershell/blob/preview/TestMappings.json#L82) (unless this file has already been removed in PR https://github.com/Azure/azure-powershell/pull/6675)