patch 8.2.0893: assert_equalfile() does not take a third argument

Problem:    Assert_equalfile() does not take a third argument.
Solution:   Implement the third argument. (Gary Johnson)
diff --git a/src/testdir/test_assert.vim b/src/testdir/test_assert.vim
index 762f12f..5037d87 100644
--- a/src/testdir/test_assert.vim
+++ b/src/testdir/test_assert.vim
@@ -81,6 +81,10 @@
   call assert_match("difference at byte 4", v:errors[0])
   call remove(v:errors, 0)
 
+  call assert_equal(1, assert_equalfile('Xone', 'Xtwo', 'a message'))
+  call assert_match("a message: difference at byte 4", v:errors[0])
+  call remove(v:errors, 0)
+
   call delete('Xone')
   call delete('Xtwo')
 endfunc