refactored the supporting file list that gets copied so it can be tested

__tests__/ImportExcelFileList.tests.ps1

@@ -0,0 +1,10 @@
+Describe "ImportExcel File List" {
+    It "All files should exist" {
+        $fileList = Get-Content "$PSScriptRoot\..\filelist.txt"
+
+        foreach ($file in $fileList) {
+            $targetFile = "$PSScriptRoot\..\$file"
+            Test-Path $targetFile | Should Be $true
+        }
+    }
+}