Add comprehensive test suite, fix minor issues, rename function and file names to match approved verbs (#708)

Scripts/Features/Backup-RegistryFeatureSelection.ps1

@@ -0,0 +1,14 @@
+<#
+    .SYNOPSIS
+        Filters a list of features to those that have a non-empty RegistryKey.
+
+    .PARAMETER Features
+        An array of feature objects to filter.
+#>
+function Get-RegistryBackedFeatures {
+    param(
+        [object[]]$Features = @()
+    )
+
+    return @($Features | Where-Object { -not [string]::IsNullOrWhiteSpace([string]$_.RegistryKey) })
+}