patch 8.2.1967: the session file does not restore the alternate file

Problem:    The session file does not restore the alternate file.
Solution:   Add ":balt".  Works like ":badd" and also sets the buffer as the
            alternate file.  Use it in the session file. (closes #7269,
            closes #6714)

src/testdir/test_buffer.vim

@@ -374,4 +374,11 @@ func Test_badd_options()
   bwipe! SomeNewBuffer
 endfunc
 
+func Test_balt()
+  new SomeNewBuffer
+  balt +3 OtherBuffer
+  e #
+  call assert_equal('OtherBuffer', bufname())
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab