moved alias test to its own test case

src/pester-tests/Get-ChildItem.Tests.ps1

@@ -1,18 +1,15 @@
 Describe "Get-ChildItem" {
     It "Should list the contents of the current folder" {
         (Get-ChildItem .).Name.Length | Should BeGreaterThan 0
-
-        (ls .).Name.Length | Should BeGreaterThan 0
     }
 
     It "Should list the contents of the home directory" {
         pushd $HOME
         (Get-ChildItem .).Name.Length | Should BeGreaterThan 0
         popd
+    }
 
-        pushd $HOME
-        (ls .).Name.Length | Should BeGreaterThan 0
-        popd
-
+    It "Should be able to use the ls alias" {
+        $(ls .).Name.Length | Should Be $(Get-ChildItem .).Name.Length
     }
 }