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_const.vim b/src/testdir/test_const.vim
index d4dccb1..d93de72 100644
--- a/src/testdir/test_const.vim
+++ b/src/testdir/test_const.vim
@@ -18,7 +18,7 @@
     const n = v:null
     const bl = 0zC0FFEE
     const here =<< trim EOS
-    hello
+      hello
     EOS
 
     call assert_true(exists('i'))
@@ -84,7 +84,7 @@
     const l:n = v:null
     const l:bl = 0zC0FFEE
     const l:here =<< trim EOS
-    hello
+      hello
     EOS
 
     call assert_fails('let l:i = 1', 'E741:')