patch 8.1.1585: :let-heredoc does not trim enough

Problem:    :let-heredoc does not trim enough.
Solution:   Trim indent from the contents based on the indent of the first
            line.  Use let-heredoc in more tests.
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index d574a8b..cd3f253 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -277,13 +277,14 @@
   if executable('cscript')
     new Xfile
     wq
-    call writefile([
-    \ 'Set fs = CreateObject("Scripting.FileSystemObject")',
-    \ 'Set ws = WScript.CreateObject("WScript.Shell")',
-    \ 'Set shortcut = ws.CreateShortcut("Xlink.lnk")',
-    \ 'shortcut.TargetPath = fs.BuildPath(ws.CurrentDirectory, "Xfile")', 
-    \ 'shortcut.Save'
-    \], 'link.vbs')
+    let lines =<< trim END
+	Set fs = CreateObject("Scripting.FileSystemObject")
+	Set ws = WScript.CreateObject("WScript.Shell")
+	Set shortcut = ws.CreateShortcut("Xlink.lnk")
+	shortcut.TargetPath = fs.BuildPath(ws.CurrentDirectory, "Xfile")
+	shortcut.Save
+    END
+    call writefile(lines, 'link.vbs')
     silent !cscript link.vbs
     call delete('link.vbs')
     call assert_equal(s:normalize_fname(getcwd() . '\Xfile'), s:normalize_fname(resolve('./Xlink.lnk')))