vim-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.
e7eb92708e

This is a missing part of Vim patch 8.1.1585 from #11211.

src/nvim/testdir/test_cindent.vim

@@ -3243,7 +3243,7 @@ func Test_cindent_30()
   setl cindent ts=4 sw=4
   setl cino=+20
 
-  let code =<< trim [CODE]
+  let code =<< [CODE]
 	void
 foo()
 {
@@ -3258,7 +3258,7 @@ func Test_cindent_30()
   normal gg
   normal ]]=][
 
-  let expected =<< trim [CODE]
+  let expected =<< [CODE]
 	void
 foo()
 {