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_balloon.vim b/src/testdir/test_balloon.vim
index 793eb3a..b3c680e 100644
--- a/src/testdir/test_balloon.vim
+++ b/src/testdir/test_balloon.vim
@@ -13,7 +13,7 @@
   throw 'Skipped: cannot make screendumps'
 endif
 
-let s:common_script =<< [CODE]
+let s:common_script =<< trim [CODE]
   call setline(1, ["one one one", "two tXo two", "three three three"])
   set balloonevalterm balloonexpr=MyBalloonExpr() balloondelay=100
   func MyBalloonExpr()