Added unit tests

Author: alan-null

Tests/MockData/auth.test.error.json

@@ -0,0 +1,4 @@
+{
+    "ok": false,
+    "error": "invalid_auth"
+}

Tests/MockData/auth.test.ok.json

@@ -0,0 +1,8 @@
+{
+    "ok": true,
+    "url": "https://subarachnoid.slack.com/",
+    "team": "Subarachnoid Workspace",
+    "user": "grace",
+    "team_id": "T12345678",
+    "user_id": "W12345678"
+}

Tests/Slack.Backup.Tests.ps1

@@ -0,0 +1,28 @@
+Clear-Host
+if (-not $PSScriptRoot) {
+    $PSScriptRoot = Split-Path $MyInvocation.MyCommand.Path -Parent
+}
+Import-Module -Name Pester -Force
+Import-Module .\Slack.Backup\Slack.Backup.psm1 -Force
+
+Describe 'Test-Auth' {
+    Context "Check token" {
+        Mock Invoke-SlackAPI -ModuleName Slack.Backup {
+            if ($Method -eq 'auth.test') {
+                $file = "auth.test.error.json"
+                if ($Token -eq "valid") {
+                    $file = "auth.test.ok.json"
+                }
+                Get-Content ".\Tests\MockData\$file" | ConvertFrom-Json
+            }
+        }
+
+        It "Given -Token '<Token>', it returns '<Expected>'" -TestCases @(
+            @{ Token = "invalid"; Expected = $false }
+            @{ Token = "valid"; Expected = $true }
+        ) {
+            param ($Token, $Expected)
+            Test-Auth -Token $Token -ErrorAction SilentlyContinue | Should -Be $Expected
+        }
+    }
+}