Add tests for TabCompletion for various input types

Add tests for TabCompletion for various input types

Related code coverage report:

https://codecov.io/gh/PowerShell/PowerShell/tree/9e41c647a9407e05d86f4971a6bd25719a7130d0/src/System.Management.Automation/engine/CommandCompletion

Environment data

> $PSVersionTable
Name                           Value
----                           -----
PSVersion                      6.0.0-beta
PSEdition                      Core
GitCommitId                    v6.0.0-beta.3
OS                             Microsoft Windows 10.0.16204
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

[daxian-dbw]

Summary of missing areas

CommandCompletion.cs

• negatives cases/error handling (if some of these can’t be hit, consider converting to asserts)
• in debugger
• nested runspace/remote runspace
• getting next result including wrap around
• user overridden tabexpansion
• legacy tabexpansion
• remove PSv2 code PSv2CompletionCompleter is for tab completion in a remote session that is from win8+ to win7, so it needs to be kept.
• adding/handling quotes

CompletionAnalysis.cs

• cursor position (you have some of these covered, not sure if all)
• filename after redirection
• line continuation
• DSC resource
• Cursor on special tokens, like comma and minus
• using module <module-name/file-completion>
• Enum property value of DSC Resource

CompletionCompleters.cs

• fully qualified cmdlet
• remove workflow code
• complete module name
• import-dscresource
• positional parameter

CompletionResult.cs

• s_nullInstance
• Exceptions

ExtensibleCompletion.cs

• Exceptions
• Register-ArgumentCompleter with CommandName = [string]::empty

PseudoParameterBinder.cs

• remove workflow related code
• examine the code to see to find out specific missing areas

[SteveL-MSFT]

I think the remaining items can be pushed to MQ