CI false signal for functional tests

[nigriMSFT]

The CI is showing as passing even though the functional.ps1 is erroring out. This error should cause CI to fail.

D:\a\win-net-test\win-net-test\tools\functional.ps1 : The variable '$RootDir' cannot be retrieved because it has not 
been set.
At D:\a\_temp\5bf[12](https://github.com/microsoft/win-net-test/actions/runs/7649151365/job/20843132494#step:6:13)8[13](https://github.com/microsoft/win-net-test/actions/runs/7649151365/job/20843132494#step:6:14)-891e-4eac-ab63-9df4ef1e02df.ps1:1 char:1
+ tools/functional.ps1 -Verbose -Config Release -Arch x64 -Iterations 1 ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (RootDir:String) [functional.ps1], RuntimeException
    + FullyQualifiedErrorId : VariableIsUndefined,functional.ps1

Example pipeline:
https://github.com/microsoft/win-net-test/actions/runs/7649151365/job/20843132494

[nigriMSFT]

FYI as seen with this experimental PR #32 , the issue only happens when the functional.ps1 script errors out due to some syntax error. Most other failures cause the CI pipeline to fail.