Msi installer: Add smoke test (#6105)

PR 6043 broke the installer (issue #6095). To prevent this from happening in the future, add a smoke test that installs the msi in appveyor build and make build fail if installation failed.
It uses the exit code to determine the success. The reason why it does not fail in the current state is because as I pointed out here, the failing custom action is not returning its exit code.

tools/appveyor.psm1

@@ -445,6 +445,7 @@ function Get-ReleaseTag
 # Implements AppVeyor 'on_finish' step
 function Invoke-AppveyorFinish
 {
+    $exitCode = 0
     try {
         $releaseTag = Get-ReleaseTag
 
@@ -474,6 +475,19 @@ function Invoke-AppveyorFinish
             $preReleaseVersion = "$previewPrefix-$previewLabel.$env:APPVEYOR_BUILD_NUMBER"
         }
 
+        # Smoke Test MSI installer
+        Write-Verbose "Smoke-Testing MSI installer" -Verbose
+        $msi = $artifacts | Where-Object { $_.EndsWith(".msi") }
+        $msiLog = Join-Path (Get-Location) 'msilog.txt'
+        $msiExecProcess = Start-Process msiexec.exe -Wait -ArgumentList "/I $msi /quiet /l*vx $msiLog" -NoNewWindow -PassThru
+        if ($msiExecProcess.ExitCode -ne 0)
+        {
+            Push-AppveyorArtifact msiLog.txt
+            $exitCode = $msiExecProcess.ExitCode
+            throw "MSI installer failed and returned error code $exitCode. MSI Log was uploaded as artifact."
+        }
+        Write-Verbose "MSI smoke test was successful" -Verbose
+
         # only publish assembly nuget packages if it is a daily build and tests passed
         if((Test-DailyBuild) -and $env:TestPassed -eq 'True')
         {
@@ -527,4 +541,13 @@ function Invoke-AppveyorFinish
     catch {
         Write-Host -Foreground Red $_
     }
+    finally {
+        # A throw statement would not make the build fail. This function is AppVeyor specific
+        # and is the only command executed in 'on_finish' phase, so it's safe that we request
+        # the current runspace to exit with the specified exit code. If the exit code is non-zero,
+        # AppVeyor will fail the build.
+        # See this link for details:
+        # https://help.appveyor.com/discussions/problems/4498-powershell-exception-in-test_script-does-not-fail-build
+        $host.SetShouldExit($exitCode)
+    }
 }