Flesh out Pester example (#403)

* Flesh out Pester example

* Remove PSScriptRoot approach

Co-authored-by: kazk <[email protected]>

Co-authored-by: kazk <[email protected]>

Author: ggorlen

content/languages/powershell/pester.md

@@ -9,6 +9,34 @@ tags:
 
 # Pester
 
+## Basic Setup
+
+### Solution Code
+
+```powershell
+function Add-Numbers($a, $b) {
+    return $a + $b
+}
+```
+
+### Test Fixture
+
+```powershell
+BeforeAll {
+    . $PSCommandPath.Replace(".Tests.ps1", ".ps1")
+}
+
+Describe "Add-Numbers" {
+    It "adds positive numbers" {
+        Add-Numbers 1 1 | Should -Be 2
+    }
+    It "adds negative numbers" {
+        Add-Numbers 1 -6 | Should -Be -5
+    }
+}
+```
+
+
 <!--
 TODO: Finish this reference
 TODO: Add tutorial and link to it