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_mksession_utf8.vim b/src/testdir/test_mksession_utf8.vim
index 7d9a909..4e593cc 100644
--- a/src/testdir/test_mksession_utf8.vim
+++ b/src/testdir/test_mksession_utf8.vim
@@ -65,32 +65,32 @@
   mksession! test_mks.out
   let li = filter(readfile('test_mks.out'), 'v:val =~# "\\(^ *normal! 0\\|^ *exe ''normal!\\)"')
   let expected =<< trim [DATA]
-  normal! 016|
-  normal! 016|
-  normal! 016|
-  normal! 08|
-  normal! 08|
-  normal! 016|
-  normal! 016|
-  normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
     normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
     normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
     normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 8 . '|'
     normal! 08|
-    exe 'normal! ' . s:c . '|zs' . 8 . '|'
     normal! 08|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
     normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
     normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
     normal! 016|
-    exe 'normal! ' . s:c . '|zs' . 16 . '|'
-    normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 8 . '|'
+      normal! 08|
+      exe 'normal! ' . s:c . '|zs' . 8 . '|'
+      normal! 08|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
+      exe 'normal! ' . s:c . '|zs' . 16 . '|'
+      normal! 016|
   [DATA]
 
   call assert_equal(expected, li)