ADUser: Refactor unit tests for resource

[johlju]

Details of the scenario you tried and the problem that is occurring

When adding unit test for new functionality in ADUser the existing test was messing up the new tests becuase they did not run it there own context and most likely left mocks that interfered with the new unit tests.

Quick solution was to add those test into a separate context block named 'When running unit tests that need refactor' in the unit tests for Set-TargetResource.

Verbose logs showing the problem

This is the output before moving the existing unit test into a separate context-block as mentioned above. The problem occur when the mock for Set-ADUser was hit.

    Context When the system is in the desired state

      Context When the parameter ThumbnailPhoto is in desired state
VERBOSE: Updating Active Directory user 'TestUser'. (ADU0012)
        [-] Should not throw and call the correct mocks 69ms
          Expected no exception to be thrown, but an exception "You cannot call a method on a null-valued expression." was thrown from C:\Program Files\WindowsPowerShell\Modules\Pester\4.8.0\Functions\Mock.ps1:1241 char:5
              +     & $cmd @BoundParameters @ArgumentList
              +     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.
          947:                         { Set-TargetResource @setTargetResourceParameters } | Should -Not -Throw
          at <ScriptBlock>, C:\Source\ActiveDirectoryDsc\Tests\Unit\MSFT_ADUser.Tests.ps1: line 947

    Context When the system is not in the desired state

      Context When the parameter ThumbnailPhoto is in desired state
VERBOSE: Updating the property 'thumbnailPhoto' with a new thumbnail photo with MD5 hash 'E3253C13DFF396BE98D6144F0DFA6105'. (ADU0022)
VERBOSE: Updating Active Directory user 'TestUser'. (ADU0012)
        [-] Should not throw and call the correct mocks 79ms
          Expected no exception to be thrown, but an exception "Exception calling "ContainsKey" with "1" argument(s): "Key cannot be null.
          Parameter name: key"" was thrown from C:\Program Files\WindowsPowerShell\Modules\Pester\4.8.0\Functions\Mock.ps1:1241 char:5
              +     & $cmd @BoundParameters @ArgumentList
              +     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.
          970:                         { Set-TargetResource @setTargetResourceParameters } | Should -Not -Throw
          at <ScriptBlock>, C:\Source\ActiveDirectoryDsc\Tests\Unit\MSFT_ADUser.Tests.ps1: line 970

Suggested solution to the issue

Refactor the unit tests to use context blocks according to unit test template (and for example unit test for ADComputer).

The DSC configuration that is used to reproduce the issue (as detailed as possible)

Not applicable.

The operating system the target node is running

Not applicable.

Version and build of PowerShell the target node is running

Not applicable.

Version of the DSC module that was used ('dev' if using current dev branch)

dev