(#956) Add initial set of Pester tests

Since we have a CLI for Chocolatey GUI, add some basic tests to ensure
it runs. This first set will only catch the most egregious errors, but
we can expand on them as we go.

Tests/chocolateyguicli.Tests.ps1

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+Import-Module helpers/gui-helpers
+
+Describe "chocolateyguicli" -Tag ChocolateyGuiCli {
+    BeforeDiscovery {
+        # Perhaps a better way is to pull these from the LiteDB similar to how we do features from the xml in CLI, but this will do for an initial setup.
+        $Features = (Invoke-GuiCli feature list -r).Lines | ConvertFrom-Csv -Delimiter '|' -Header Name, state, description | Select-Object Name, @{Name = 'enabled'; Expression = { $_.state -eq 'Enabled' } }
+        $Features | Out-string | write-host
+    }
+
+    Context "Basic CLI functionality" {
+        BeforeAll {
+            $Output = Invoke-GuiCli
+        }
+
+        It 'Should exit Success (0)' {
+            $Output.ExitCode | Should -Be 0 -Because $Output.String
+        }
+
+        It 'Should output appropriate message' {
+            $Output.Lines | Should -Contain "Please run chocolateyguicli with 'chocolateyguicli -?' or 'chocolateyguicli <command> -?' for specific help on each command"
+        }
+    }
+
+    Context "Lists available features" {
+        BeforeAll {
+            $Output = Invoke-GuiCli feature list
+        }
+
+        It "Contains reference of the option (<_.Name>)" -ForEach $Features {
+            $Output.String | Should -Match $Name
+        }
+    }
+
+    Context "Toggles the feature (<_.Name>) successfully" -ForEach $Features {
+        BeforeAll {
+            $Outputs = if ($Enabled) {
+                Invoke-GuiCli feature disable --name $_.Name
+                Invoke-GuiCli feature enable --name $_.Name
+            } else {
+                Invoke-GuiCli feature enable --name $_.Name
+                Invoke-GuiCli feature disable --name $_.Name
+            }
+        }
+
+        It "Should exit success" {
+            $Outputs.ExitCode | Should -Not -Contain 1
+        }
+    }
+}

Tests/helpers/gui-helpers.psm1

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+Get-ChildItem -Path $PSScriptRoot\gui -Filter *.ps1 -Recurse | ForEach-Object { . $_.FullName }

Tests/helpers/gui/Invoke-GuiCli.ps1

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+function Invoke-GuiCli {
+    <#
+        .Synopsis
+            Helper function to call chocolatey with any number of specified arguments,
+            and return a hashtable with the output as well as the exit code.
+    #>
+    [CmdletBinding()]
+    param(
+        # The arguments to use when calling the Choco executable
+        [Parameter(Position = 1, ValueFromRemainingArguments)]
+        [string[]]$Arguments
+    )
+
+    $output = & chocolateyguicli.exe @arguments
+    [PSCustomObject]@{
+        # We trim all the lines, so we do not take into account
+        # trimming the lines when asserting, and that extra whitespace
+        # is not considered in our assertions.
+        Lines    = if ($output) { $output.Trim() } else { @() }
+        String   = $output -join "`r`n"
+        ExitCode = $LastExitCode
+    }
+}