Fix Installer Builds: Test in a separate step

[papeh]

Also remove the redundant Platform from CD.yml

Summary

Calling test.ps1 from build.ps1 throws an error:

Running tests...
Invoking test runner with arguments: -Configuration; Release; -NoBuild; -Verbosity; minimal; -TestFilter; TestCategory!=ByHand&TestCategory!=SmokeTest&TestCategory!=DesktopRequired
C:\fwrepo\fw\build.ps1 : Cannot validate argument on parameter 'Verbosity'. The argument "-Verbosity" does not belong to the set "quiet,minimal,normal,detailed,q,m,n,d" specified by the ValidateSet attribute. Supply an argument that is in the set and then try the
command again.
At line:1 char:1
+ .\build.ps1 -Configuration Release -RunTests -TestFilter 'TestCategor ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [build.ps1], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationError,build.ps1

I'm not sure what is causing Powershell to misinterpret the args, but I see no way around it.

Use a separate step (as does CI.yml) rather than specifying /p:action=test as installer builds used to, because this
seems to be the way of the future.

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This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   5m 36s ⏱️ -56s
4 407 tests ±0  4 320 ✅ ±0  87 💤 ±0  0 ❌ ±0 
4 416 runs  ±0  4 329 ✅ ±0  87 💤 ±0  0 ❌ ±0 

Results for commit affba58. ± Comparison against base commit 7821251.

♻️ This comment has been updated with latest results.