Add Pester V5 Agent Level Checks

[ClaudioESSilva]

Introducing Agent checks to Pester V5.
Each check needs rewritten and added to \checks\Agentv5.Tests.ps1

• DatabaseMailEnabled
• AgentServiceAccount
• DbaOperator
• FailsafeOperator
• DatabaseMailProfile
• AgentMailProfile
• FailedJob
• ValidJobOwner
• InValidJobOwner
• AgentAlert
• JobHistory
• LongRunningJob
• LastJobRunTime