patch 8.1.1362: code and data in tests can be hard to read

Problem:    Code and data in tests can be hard to read.
Solution:   Use the new heredoc style. (Yegappan Lakshmanan, closes #4400)
diff --git a/src/testdir/test_balloon.vim b/src/testdir/test_balloon.vim
index 57d8dcc..39c2602 100644
--- a/src/testdir/test_balloon.vim
+++ b/src/testdir/test_balloon.vim
@@ -8,14 +8,14 @@
   finish
 endif
 
-let s:common_script = [
-	\ 'call setline(1, ["one one one", "two tXo two", "three three three"])',
-	\ 'set balloonevalterm balloonexpr=MyBalloonExpr() balloondelay=100',
-	\ 'func MyBalloonExpr()',
-	\ ' return "line " .. v:beval_lnum .. " column " .. v:beval_col .. ": " .. v:beval_text',
-	\ 'endfun',
-	\ 'redraw',
-	\ ]
+let s:common_script =<< [CODE]
+  call setline(1, ["one one one", "two tXo two", "three three three"])
+  set balloonevalterm balloonexpr=MyBalloonExpr() balloondelay=100
+  func MyBalloonExpr()
+    return "line " .. v:beval_lnum .. " column " .. v:beval_col .. ": " .. v:beval_text
+  endfun
+  redraw
+[CODE]
 
 func Test_balloon_eval_term()
   " Use <Ignore> after <MouseMove> to return from vgetc() without removing