setup tests: conditionally test default installation

Default installation (no arguments or preexisting environment variables)
can be tested only when the test machine does not already have Chocolatey
installed in the default location (otherwise it would get overwritten).

To enable these tests, make sure your Chocolatey is installed in a custom
location.

tests/unit/Initialize-Chocolatey.Tests.ps1

@@ -59,10 +59,62 @@ function Setup-ChocolateyInstallationPackage
     Import-Module "$tmpDir\chocolateysetup.psm1"
 }
 
+function Get-DefaultChocolateyInstallDir
+{
+    $programData = [Environment]::GetFolderPath([Environment+SpecialFolder]::CommonApplicationData)
+    $chocolateyPath = Join-Path $programData chocolatey
+    return $chocolateyPath
+}
+
+function Execute-ChocolateyInstallationInDefaultDir($scriptBlock)
+{
+    $defaultDir = Get-DefaultChocolateyInstallDir
+    if (Test-Path $defaultDir) {
+        Write-Warning "Skipping default installation test because the default installation directory already exists ($defaultDir)"
+        return
+    }
+    $script:installDir = $defaultDir
+    try
+    {
+        & $scriptBlock
+    }
+    finally
+    {
+        Write-Debug "Removing default installation directory if exists ($defaultDir)"
+        Get-Item $defaultDir | Remove-Item -Recurse -Force
+    }
+}
+
 Describe "Initialize-Chocolatey" {
     # note: the specs below are correct when installing with administrative permissions with UAC enabled
     # the test suite is always run elevated, so no easy way to test limited user install for now
 
+    Context "When installing with `$Env:ChocolateyInstall not set and no arguments" {
+        Setup-ChocolateyInstallationPackage
+
+        Execute-WithEnvironmentBackup {
+            Setup-ChocolateyInstall $null
+
+            Execute-ChocolateyInstallationInDefaultDir {
+                Initialize-Chocolatey
+
+                Verify-ExpectedContentInstalled $installDir
+
+                It "should create ChocolateyInstall at Process scope" {
+                    Assert-ChocolateyInstallIs $installDir 'Process'
+                }
+
+                It "should create ChocolateyInstall at User scope" {
+                    Assert-ChocolateyInstallIs $installDir 'User'
+                }
+
+                It "should not create ChocolateyInstall at Machine scope" {
+                    Assert-ChocolateyInstallIsNull 'Machine'
+                }
+            }
+        }
+    }
+
     Context "When installing with `$Env:ChocolateyInstall not set, with explicit chocolateyPath" {
         Setup-ChocolateyInstallationPackage