Added Test-IsNanoServer - Fixes #9

[PlagueHO]

This PR adds Test-IsNanoServer, using the code as suggested in this issue:
dsccommunity/xPSDesiredStateConfiguration#652

This allows for a low risk way of moving xPSDesiredStateConfiguration a faster method of Test-IsNanoServer.

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This change is 

[johlju]

Reviewed 6 of 6 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[johlju]

Using Reviewable from the phone actually worked 😃

[johlju]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @PlagueHO)

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tests/Unit/Public/Test-IsNanoServer.Tests.ps1, line 18 at r1 (raw file):

Mock -CommandName Get-CimInstance

THis cmdlet does not exist on macOS and Linux, so either we skip the test or we mock it using a fake function function Get-CimInstance {} to pass on all OS. I suggest we skip the test on unsupported platforms.

[johlju]

@PlagueHO I missed that the tests failed yesterday.

[PlagueHO]

Should be good to go now @johlju

[johlju]

Reviewed 3 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved