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_let.vim b/src/testdir/test_let.vim
index 4fa010b..7a547ce 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -210,10 +210,18 @@
 	END
   call assert_equal(['Line1', '  Line2', "\tLine3", ' END'], var1)
 
+  let var1 =<< trim !!!
+	Line1
+	 line2
+		Line3
+	!!!
+  !!!
+  call assert_equal(['Line1', ' line2', "\tLine3", '!!!',], var1)
+
   let var1 =<< trim
     Line1
   .
-  call assert_equal(['  Line1'], var1)
+  call assert_equal(['Line1'], var1)
 
   " ignore "endfunc"
   let var1 =<< END