patch 9.0.1300: 'statusline' only supports one "%=" item

Problem:    'statusline' only supports one "%=" item.
Solution:   Add support for multiple "%=" items. (TJ DeVries, Yegappan
            Lakshmanan, closes #11970, closes #11965)
diff --git a/src/testdir/test_statusline.vim b/src/testdir/test_statusline.vim
index 8061d15..1239e4e 100644
--- a/src/testdir/test_statusline.vim
+++ b/src/testdir/test_statusline.vim
@@ -231,6 +231,10 @@
   " %=: Separation point between left and right aligned items.
   set statusline=foo%=bar
   call assert_match('^foo\s\+bar\s*$', s:get_statusline())
+  set statusline=foo%=bar%=baz
+  call assert_match('^foo\s\+bar\s\+baz\s*$', s:get_statusline())
+  set statusline=foo%=bar%=baz%=qux
+  call assert_match('^foo\s\+bar\s\+baz\s\+qux\s*$', s:get_statusline())
 
   " Test min/max width, leading zeroes, left/right justify.
   set statusline=%04B